Discovery and first devices

The phenomenon of electroluminescence was discovered in 1907 by British experimenter Henry Joseph Round of the Marconi Laboratories, using a silicon carbide crystal and a cat's whisker detector. Soviet inventor Oleg Losev reported the construction of the first LED in 1927. His research appeared in Soviet, German, and British scientific journals, but the discovery was not put into practice until several decades later. Kurt Lehovec, Carl Accardo and Edward Jamgochian interpreted the mechanism of these first LED diodes in 1951, using an apparatus that used silicon carbide crystals, with a pulse generator and a power supply, and in 1953 with a pure variant of the crystal.

Rubin Braunstein of RCA reported in 1955 on the infrared emission from gallium arsenide (GaAs) and other semiconductor alloys. Braunstein observed that this emission was generated in diodes constructed from alloys of gallium antimonide (GaSb), gallium arsenide (GaAs), indium phosphide (InP) and silicon-germanium (SiGe) at room temperature and 77 kelvin.

In 1957, Braunstein also demonstrated that these rudimentary devices could be used to establish non-radio communication over short distances. As Kroemer points out, Braunstein established a very simple line of optical communications:[18]​ he took music from a record player and processed it using the appropriate electronics to modulate the direct current produced by a GaAs gallium arsenide diode. The light emitted by the GaAS diode was able to sensitize a PbS lead sulfide "Lead(II) sulfide") diode located at a certain distance. The signal thus generated by the PbS diode was introduced into an audio amplifier and transmitted through a speaker. When the light beam was intercepted between the two LEDs, the music stopped. This assembly already foreshadowed the use of LEDs for optical communications.

In September 1961, James R. Biard and Gary Pittman, working at Texas Instruments (TI) in Dallas, Texas, discovered infrared radiation (900 nm) from a tunnel diode they had constructed using a gallium arsenide (GaAs) substrate.[19] In October 1961, they demonstrated efficient light emissions and signal coupling between the arsenide p-n junction. of light-emitting gallium and an electrically insulated photodetector constructed from a semiconductor material.[20]​ Based on their discoveries, on August 8, 1962, Biard and Pittman produced a patent titled “Semiconductor Radiant Diode”[21]​ that described how a zinc alloy diffused during crystal growth that forms the substrate of a p-n LED junction with a sufficiently spaced cathode contact, allowed Efficient infrared light emission in direct polarization.

In view of the importance of their research, as it appeared in their engineering notebooks and even before communicating their results from the laboratories of General Electric, Radio Corporation of America, IBM, Bell Laboratories or those of the Lincoln Laboratory of the Massachusetts Institute of Technology, the United States Patent and Trademark Office granted them a patent for the invention of gallium arsenide infrared light-emitting diodes (patent US3293513A of the USA),[22]​ which are considered the first LEDs for practical use. Immediately after the patent was filed, TI began a project to manufacture the infrared diodes. In October 1962, Texas Instruments developed the first commercial LED (the SNX-100), which used a pure gallium arsenide crystal to emit 890 nm light. In October 1963, TI launched the first commercial hemispherical LED, the SNX-110.[23]​.

The first LED with emission in the visible (red) spectrum was developed in 1962 by Nick Holonyak Jr. when he worked at General Electric. Holonyak reported in the journal Applied Physics Letters on December 1, 1962. telecommunications through optical fibers. To this end, he discovered new semiconductor materials expressly adapted to the wavelengths of the aforementioned transmission through optical fibers.[26]​.

Initial business development

The first commercial LEDs were generally used to replace incandescent lamps and neon indicator lamps as well as in seven-segment displays.[27] First in expensive equipment such as electronic and laboratory test equipment, and later in other electrical devices such as televisions, radios, telephones, calculators, as well as wristwatches. Until 1968, visible and infrared LEDs were extremely expensive, on the order of $200 per unit, so they had little practical use.[28]​ The Monsanto Company was the first to mass produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs for indicators.[28]​.

Hewlett-Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. These red LEDs were bright enough to be used as indicators, since the light emitted was not enough to illuminate an area. The readings on the calculators were so weak that plastic lenses were placed over each digit to make them legible. Later, other colors appeared and were widely used in gadgets and equipment. In the 1970s Fairchild Optoelectronics manufactured commercially successful LED devices for less than five cents each. These devices used compound semiconductor chips manufactured using the planar process invented by Jean Hoerni of Fairchild Semiconductor.[29]​[30]​ Planar processing for chip manufacturing combined with innovative encapsulation methods allowed the team led by optoelectronics pioneer Thomas Brandt to achieve the cost reductions needed at Fairchild.[31]​ These methods continue to be used by LED manufacturers.[32]​.

Most LEDs were manufactured in the typical 5mm T1¾ and 3mm T1 packages, but with increasing power output, it has become increasingly necessary to remove excess heat to maintain reliability.[33]​ It has therefore been necessary to design more complex packages designed to achieve efficient heat dissipation. The encapsulations currently used for high-power LEDs bear little resemblance to those of the first LEDs.

blue led

Blue LEDs were first developed by Henry Paul Maruska of RCA in 1972 using Gallium Nitride (GaN) on a sapphire substrate.[34]​[35]​

SiC type LEDs (made with silicon carbide) began to be marketed by Cree, Inc., United States in 1989.[36] However, none of these blue LEDs were very bright.

The first high-brightness blue LED was presented by Shuji Nakamura of Nichia Corp. in 1994, starting from the material Indium-Gallium Nitride (InGaN). For their research, Nakamura, Akasaki and Amano were awarded the Nobel Prize in Physics.[39][40] In 1995, Alberto Barbieri from the Cardiff University laboratory (UK) investigated the efficiency and reliability of high-brightness LEDs and as a result of the research obtained an LED with a transparent contact electrode using indium tin oxide (ITO) on aluminum-gallium-indium phosphide and gallium arsenide.

In 2001[41]​ and 2002[42]​ processes were carried out to grow gallium nitride LEDs on silicon. As a consequence of these investigations, in January 2012 Osram launched high-power gallium-indium nitride LEDs grown on a silicon substrate.[43]​.

White LED and evolution

The achievement of high efficiency in blue LEDs was quickly followed by the development of the first white LED. In such a device a Y Al O:Ce coating “phosphor” (fluorescent material) (known as YAG or yttrium aluminum garnet) absorbs some of the blue emission and generates yellow light by fluorescence. In a similar way, it is possible to introduce other “matches” that generate green or red light by fluorescence. The resulting mixture of red, green and blue is perceived by the human eye as white; On the other hand, it would not be possible to appreciate red or green objects by illuminating them with the YAG phosphor since it generates only yellow light along with a remnant of blue light.

The first white LEDs were expensive and inefficient. However, the intensity of light produced by LEDs has increased exponentially, with a doubling time occurring approximately every 36 months since the 1960s (according to Moore's Law). This trend is generally attributed to a parallel development of other semiconductor technologies and advances in optics and materials science, and has been named Haitz's law in honor of Roland Haitz.[44]

The light output and efficiency of blue and near-ultraviolet LEDs increased while the cost of lighting devices manufactured with them decreased, which led to the use of white light LEDs for lighting. The fact is that they are replacing incandescent and fluorescent lighting.[45][46]​.

White LEDs can produce 300 lumens per electrical watt while lasting up to 100,000 hours. Compared to incandescent bulbs, this represents not only a huge increase in electrical efficiency, but also a similar or lower cost per bulb.[47]​.

A P-N junction can provide an electric current when illuminated. Similarly, a P-N junction crossed by a direct current can emit light photons. There are two ways of considering the phenomenon of electroluminescence. In the second case, this could be defined as the emission of light by a semiconductor when it is subjected to an electric field. The charge carriers recombine in a forward-biased P-N junction. Specifically, the electrons in the N region cross the potential barrier and recombine with the holes in the P region. The free electrons are in the conduction band while the holes are in the valence band. In this way, the energy level of holes is lower than that of electrons. When electrons and holes recombine, a fraction of the energy is emitted in the form of heat and another fraction in the form of light.

The physical phenomenon that takes place in a PN junction when the current passes in forward polarization, therefore, consists of a succession of electron-hole recombinations. The phenomenon of recombination is accompanied by the emission of energy. In ordinary Germanium or Silicon diodes, phonons or vibrations of the crystalline structure of the semiconductor are produced that simply contribute to its heating. In the case of LED diodes, the semiconductor materials are different from the previous ones, for example, being various type III-V alloys such as gallium arsenide (AsGa), gallium phosphide") (PGa) or gallium phosphoarsenide (PAsGa).

In these semiconductors, the recombinations that develop in the PN junctions eliminate excess energy by emitting light photons. The color of the emitted light depends directly on its wavelength and is characteristic of each specific alloy. Currently, alloys are manufactured that produce luminous photons with wavelengths in a wide range of the electromagnetic spectrum within the visible, near-infrared and near-ultraviolet. What is achieved with these materials is to modify the energy width of the bandgap, thus modifying the wavelength of the emitted photon. If the LED diode is reverse polarized, the recombination phenomenon will not occur, so it will not emit light. Reverse bias can damage the diode.

The electrical behavior of the LED diode in forward polarization is as follows. If the polarization voltage is increased, from a certain value (which depends on the type of semiconductor material), the LED begins to emit photons, the turn-on voltage has been reached. Electrons can move across the junction by applying different voltages to the electrodes; Thus, the emission of photons begins and as the polarization voltage increases, the intensity of the emitted light increases. This increase in luminous intensity is coupled with the increase in current intensity and can be decreased by Auger recombination. During the recombination process, the electron jumps from the conduction band to the valence band, emitting a photon and accessing, by conservation of energy and momentum, a lower energy level, below the Fermi level of the material. The emission process is called radiative recombination, which corresponds to the phenomenon of spontaneous emission. Thus, in each electron-hole radiative recombination, a photon of energy equal to the width in energies of the bandgap is emitted:.

where c is the speed of light and f and λ are the frequency and wavelength, respectively, of the light it emits. This description of the basis of the emission of electromagnetic radiation by the LED diode can be seen in the figure where a schematic representation of the PN junction of the semiconductor material is made together with the energy diagram, involved in the recombination process and light emission, in the lower part of the drawing. The wavelength of the emitted light, and therefore its color, depends on the width of the energy bandgap. The most important substrates available for application in light emission are GaAs and InP. LED diodes can reduce their efficiency if their absorption and spectral emission peaks depending on their wavelength are very close, as is the case with GaAs:Zn (zinc-doped gallium arsenide) LEDs, since part of the light they emit is absorbed internally.

The materials used for LEDs have a band gap in direct polarization whose width in energies varies from infrared light to visible light or even near ultraviolet light. The evolution of LEDs began with infrared and red gallium arsenide devices. Advances in materials science have made it possible to manufacture devices with increasingly shorter wavelengths, emitting light in a wide range of colors. LEDs are generally fabricated on an N-type substrate, with an electrode connected to the P-type layer deposited on its surface. P-type substrates, although less common, are also manufactured.

Physical foundation

An LED begins to emit when a voltage of 2-3 volts is applied to it. In reverse polarization, a different vertical axis is used than in forward polarization to show that the current absorbed is practically constant with the voltage until breakdown occurs.

The LED is a diode made up of a semiconductor chip doped "Doping (semiconductors)") with impurities that create a PN junction. As in other diodes, current flows easily from the p side, or anode, to the n side, or cathode, but not in the opposite direction. Charge carriers (electrons and holes) flow into the junction from two electrodes set at different voltages (voltage (electricity)). When an electron recombines with a hole, its energy level drops and the excess energy is released in the form of a photon. The wavelength of the emitted light, and therefore the color of the LED, depends on the energy width of the bandgap corresponding to the materials that make up the pn junction.

In silicon or germanium diodes, electrons and holes recombine, generating a non-radiative transition, which does not produce any light emission, since they are semiconductor materials with an indirect bandgap. The materials used in LEDs have a direct bandgap with an energy width that corresponds to the light spectrum of the near-infrared (800-2500 nm), the visible and the near-ultraviolet (200-400 nm).

The development of LEDs began with red and infrared light devices, made with gallium arsenide (GaAs). Advances in materials science have made it possible to build devices with increasingly smaller wavelengths, emitting light within a wide range of colors.

LEDs are typically made from an n-type substrate, with one of the electrodes bonded to the p-type layer deposited on its surface. p-type substrates are also used, although they are less common. Many commercial LEDs, especially GaN/InGaN, also use sapphire (aluminum oxide) as a substrate.

Most of the semiconductor materials used in the manufacture of LEDs have a very high refractive index. This implies that the majority of the light emitted inside the semiconductor is reflected when it reaches the outer surface that is in contact with the air by a phenomenon of total internal reflection. The extraction of light constitutes, therefore, a very important aspect and one in constant research and development to take into consideration in the production of LEDs.

refractive index

Most of the semiconductor materials used in the manufacture of LEDs have a very high refractive index with respect to air. This implies that the majority of the light emitted inside the semiconductor will be reflected when it reaches the outer surface that is in contact with the air by a phenomenon of total internal reflection.

This phenomenon affects both the light emission efficiency of the LEDs and the light absorption efficiency of the photovoltaic cells. The refractive index of silicon is 3.96 (at 590 nm),[48]​ while that of air is 1.0002926.[48]​ Light extraction constitutes, therefore, a very important aspect and one in constant research and development to take into consideration in the production of LEDs.

In general, an uncoated flat surface LED semiconductor chip will emit light only in the direction perpendicular to the surface of the semiconductor and in very close directions, forming a cone called light cone[49]​ or escape cone.[50]​ The maximum angle of incidence that allows photons to escape from the semiconductor is known as the critical angle. When this angle is exceeded, the photons no longer escape from the semiconductor, but instead are reflected within the semiconductor crystal as if there were a mirror on the outer surface.[50]​.

Due to internal reflection, light that has been reflected internally on one face can escape through other crystal faces if the angle of incidence now becomes sufficiently low and the crystal is sufficiently transparent not to reflect the photon emission back inward. However, in a simple cubic LED with external surfaces at 90 degrees, all faces act as equal angled mirrors. In this case, most of the light cannot escape and is lost as heat within the semiconductor crystal.[50]​.

A chip that has angled facets on its surface similar to those of a cut jewel or a Fresnel lens can increase light output by allowing its emission in orientations that are perpendicular to the outer facets of the chip, typically more numerous than the only six in a cubic sample.[51]​.

The ideal shape of a semiconductor to obtain the maximum light output would be that of a microsphere") with the emission of the photons located exactly in the center of it, and equipped with electrodes that penetrate to the center to connect with the point of emission. All the light rays that start from the center would be perpendicular to the surface of the sphere, which would result in no internal reflections. A hemispherical semiconductor would also work correctly since the flat part would act as a mirror to reflect the photons so that all light could be emitted completely through the hemisphere.[52]​.

After building a wafer "Wafer (electronic)") of semiconductor material, it is cut into small fragments. Each fragment is called a chip and becomes the small active part of a light-emitting LED diode.

Many LED semiconductor chips are encapsulated or incorporated inside molded plastic housings. The plastic casing aims to achieve three purposes:

1. • Facilitate the assembly of the semiconductor chip in lighting devices.

2. • Protect the fragile electrical wiring associated with the diode from physical damage.

3. • Act as an intermediary element for the refraction effect between the high index of the semiconductor and that of the air.

The third feature contributes to increasing the light emission from the semiconductor by acting as a diffuser lens, allowing light to be emitted to the outside with an angle of incidence on the outer wall much greater than that of the narrow cone of light coming from the uncoated chip.

Efficiency and operational parameters

LEDs are designed to operate with an electrical power no greater than 30-60 milliwatts (mW). Around 1999, Philips Lumileds introduced more powerful LEDs capable of working continuously at a power of one watt. These LEDs used much larger die-cut semiconductors in order to accept higher power supplies. Additionally, they were mounted on metal rods to facilitate heat removal.

One of the main advantages of LED-based lighting sources is the high luminous efficiency. White LEDs quickly matched and even surpassed the efficiency of standard incandescent lighting systems. In 2002, Lumileds manufactured five-watt LEDs, with a luminous efficiency of 18-22 lumens per watt (lm/W). For comparison, a conventional 60-100 watt incandescent bulb emits around 15 lm/W, and standard fluorescent lamps emit up to 100 lm/W.

As of 2012, Future Lighting Solutions") had achieved the following efficiencies for some colors.[53] Efficiency values ​​show the luminous power output per watt of input electrical power. The luminous efficiency values ​​include the characteristics of the human eye and have been deduced from the luminosity function.

In September 2003, Cree Inc." manufactured a new type of blue LED that consumed 24 milliwatts (mW) at 20 milliamps (mA). This enabled a new encapsulation of white light that produced 65 lm/W at 20 milliamps, making it the brightest white LED available on the market; it was also more than four times more efficient than standard incandescent bulbs. In 2006 they introduced a white LED prototype with a record luminous efficiency of 131 lm/W for a current of 20 milliamps. Nichia Corporation") has developed a white LED with a luminous efficiency of 150 lm/W and a direct current of 20 mA.[54] The LEDs from the company Cree Inc. called xlamp xm-L, came onto the market in 2011, producing 100 lm/W at the maximum power of 10 W, and up to 160 lm/W with an input electrical power of about 2 W. In 2012, Cree Inc. introduced a white LED capable of producing 254 lm/W,[55]​ and 303 lm/W in March 2014.[56]​ General lighting needs in practice require high-power LEDs, one watt or more. They operate with currents greater than 350 milliamps.

These efficiencies refer to the light emitted by the diode kept at a low temperature in the laboratory. Since LEDs, once installed, operate at high temperatures and with conduction losses, the efficiency is actually much lower. The United States Department of Energy (DOE) has carried out tests to replace incandescent lamps or CFLs with LED lamps, showing that the average efficiency achieved is about 46 lm/W in 2009 (the behavior during the tests remained within a range of 17 lm/W to 79 lm/W).[57]​.

When the electric current supplied to an LED exceeds a few tens of milliamps, the luminous efficiency decreases due to an effect called loss of efficiency.

At first, an explanation was sought attributing it to high temperatures. However, scientists were able to demonstrate the opposite, that although the life of the LED may be shortened, the drop in efficiency is less severe at elevated temperatures.[58]​ In 2007, the cause of the decrease in efficiency was attributed to Auger recombination") which gives rise to a mixed reaction.[59]​ Finally, a 2013 study definitively confirmed this theory to justify the loss of efficiency.[60]​.

Half-life and failure analysis

Solid state devices such as LEDs have very limited obsolescence if operated at low currents and low temperatures. Life times are 25,000 to 100,000 hours, but the influence of heat and current can increase or decrease this time significantly.[66]​.

The most common failure of LEDs (and laser diodes) is the gradual reduction of light output and loss of efficiency. The first red LEDs stood out for their short life. With the development of high-power LEDs, devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress in the material and can cause early degradation of light output. To quantitatively classify the useful life in a standardized way, it has been suggested to use the L70 or L50 parameters, which represent the life times (expressed in thousands of hours) in which a given LED reaches 70% and 50% of the initial light emission, respectively.[67]​.

Just as in most previous light sources (incandescent lamps, discharge lamps, and those that burn a fuel, for example candles and oil lamps) the light was generated by a thermal process, LEDs only work correctly if they are kept sufficiently cool. The manufacturer usually specifies a maximum junction temperature between 125 and 150°C, and lower temperatures are recommended in the interest of achieving long life for the LEDs. At these temperatures, relatively little heat is lost through radiation, meaning that the light beam generated by an LED is considered cold.

The waste heat in a high-power LED (which as of 2015 can be considered less than half the electrical power it consumes) is transported by conduction through the substrate and encapsulation to a heat sink, which removes the heat into the environment by convection. It is therefore essential to carry out a careful thermal design, taking into account the thermal resistances of the LED encapsulation, the heat sink and the interface between the two. Medium power LEDs are typically designed to be soldered directly to a printed circuit board that has a thermally conductive metal layer. High power LEDs are encapsulated in large surface area ceramic packages designed to be connected to a metal heat sink, the interface being a material with high thermal conductivity (thermal paste, phase change material, conductive thermal pad") or hot melt glue).

If an LED lamp is installed in a lighting fixture without ventilation, or the environment lacks fresh air circulation, the LEDs are likely to overheat, reducing their lifespan or even causing early deterioration of the lighting fixture. Thermal design is typically designed for an ambient temperature of 25°C (77°F). LEDs used in outdoor applications, such as traffic signs or pavement marking lights, and in climates where the temperature inside the lighting fixture is very high, can experience anything from reduced luminous output to complete failure.[68]​.

Contenido

Los ledes convencionales están fabricados a partir de una gran variedad de materiales semiconductores inorgánicos. En la siguiente tabla se muestran los colores disponibles con su margen de longitudes de onda, diferencias de potencial de trabajo y materiales empleados.

Blue and ultraviolet

The first blue-violet LED used magnesium-doped chlorine and was developed by Herb Maruska and Wally Rhines at Stanford University in 1972, PhD students in materials science and engineering.[82][83] At that time Maruska was working in the RCA laboratories, where he collaborated with Jacques Pankove. In 1971, a year after Maruska left for Stanford, his RCA colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc doped with gallium nitride; However, the device Pankove and Miller built, the first true gallium nitride light-emitting diode, emitted green light.[84] In 1974 the US Patent Office granted Maruska, Rhines, and Stanford professor David Stevenson a patent (US patent US3819974 A)[85] for their 1972 work on doping gallium nitride with magnesium that today continues to be the basis of all commercial blue LEDs and laser diodes. These devices built in the 1970s did not have sufficient luminous output for practical use, so research into gallium nitride diodes slowed down. In August 1989, Cree introduced the first commercial blue LED with an indirect transition through the bandgap in a silicon carbide (SiC) semiconductor.[86]​[87]​ SiC LEDs have a very low luminous efficiency, no more than 0.03%, but emit in the visible blue region.

In the late 1980s, breakthroughs in epitaxial growth and p-type doping in GaN ushered in the modern era of GaN opto-electronic devices. Based on the above, Theodore Moustakas patented a method of producing blue LEDs at Boston University using a novel two-step process.[89] Two years later, in 1993, high-intensity blue LEDs were taken up by Shuji Nakamura of the Nichia Corporation using GaN synthesis processes similar to Moustakas'.[90] Moustakas and Nakamura were assigned separate patents, leading to legal conflicts between them. Nichia and Boston University (especially since, although Moustakas invented his process first, Nakamura registered his before).[91] This new development revolutionized LED lighting, making the manufacture of high-power blue light sources more profitable, leading to the development of technologies such as Blu-ray, and enabling the bright, high-resolution screens of modern tablets and phones.

Nakamura was awarded the Millennium Technology Prize for his contribution to high-power LED technology and high performance.[92] He was also awarded, along with Hiroshi Amano and Isamu Akasaki, the Nobel Prize in Physics in 2014 for his decisive contribution to high-performance LEDs and blue LEDs.[93][94][95][96] In 2015, a US court ruled that three companies (that is, the same plaintiff companies that had not previously resolved their disputes) and that held Nakamura's patents for production in the US, had infringed Moustakas' previous patent and ordered them to pay licensing fees worth $13 million.[97]

By the end of the 90s, blue LEDs were already available. These feature an active region consisting of one or more InGaN quantum wells sandwiched between thicker sheets of GaN, called sheaths. By varying the In/Ga fraction in the InGaN quantum wells, the light emission can, in theory, be shifted from violet to amber. AlGaN gallium aluminum nitride with varying content of the Al/Ga fraction can be used to fabricate the sheath and sheets of quantum wells for UV diodes, but these devices have not yet reached the level of efficiency and technological maturity of blue/green InGaN/GaN devices. If the GaN is used without doping, to form the active layers of the quantum wells the device emits light close to ultraviolet with a peak centered at a wavelength around 365 nm. Green LEDs manufactured in the InGaN/GaN mode are much more efficient and brighter than LEDs produced with non-nitride systems, but these devices still have too low an efficiency for high-brightness applications.

Using aluminum nitrides, such as AlGaN and AlGaInN, even shorter wavelengths are achieved. A range of UV LEDs for different wavelengths are beginning to become available on the market. Near-UV emitting LEDs with wavelengths around 375-395 nm are already sufficiently cheap and can be easily found, for example to replace black light lamps in the inspection of UV anti-counterfeiting watermarks on some documents and on paper money. Shorter wavelength diodes (up to 240 nm),[98]​ are currently on the market, although they are noticeably more expensive.

As the photosensitivity of microorganisms approximately coincides with the absorption spectrum of DNA (with a peak around 260 nm), it is expected to use UV LEDs with emission in the region of 250-270 nm in disinfection and sterilization equipment. Recent research has shown that commercially available UV LEDs (365 nm) are effective in disinfection and sterilization devices.[99]​ UV-C wavelengths were obtained in laboratories using aluminum nitride (210 nm), boron nitride (215 nm), and diamond (235 nm).

RGB

RGB LEDs consist of a red, a blue and a green LED. By independently adjusting each of them, RGB LEDs are capable of producing a wide range of colors. Unlike LEDs dedicated to a single color, RGB LEDs do not produce pure wavelengths. Additionally, commercially available modules are typically not optimized for smooth color blends.

RGB systems

RGB systems.

There are two basic ways to produce white light. One is to use individual LEDs that emit the three primary colors (red, green and blue) and then mix the colors to form white light. The other way is to use a phosphor to convert the monochromatic light from a blue or UV LED into a broad spectrum of white light. It is important to keep in mind that the whiteness of the light produced is essentially designed to satisfy the human eye and depending on each case it may not always be appropriate to think that it is strictly white light. The great variety of whites that are achieved with fluorescent tubes serves as a point of reference.

There are three main methods of producing white light with LEDs.

• - Blue LED + green LED + red LED (mix of colors; although it can be used as a backlight for screens) for lighting they are very poor due to the empty intervals in the frequency spectrum).

• - Near UV LED or UV + RGB phosphor (an LED light that generates a shorter wavelength than blue is used to excite an RGB phosphor).

• - Blue LED + yellow phosphor (two complementary colors combine to produce white light; it is more efficient than the first two methods and is therefore more used in practice).

Due to metamerism "Metamerism (color)"), it is possible to have different spectra that appear white. However, the appearance of objects illuminated by that light can be modified as the spectrum varies. This optical phenomenon is known as color execution, it is different from color temperature, and it makes a truly orange or cyan object appear to be another color and much darker as the LED or the associated phosphor does not emit those wavelengths. The best color reproduction with CFL and LED is achieved by using a mixture of phosphors, which provides lower efficiency, but better light quality. Although the halogen with the highest color temperature is orange, it is still the best artificial light source available in terms of color execution.

White light can be produced by adding lights of different colors; The most common method is the use of red, green and blue (RGB). Hence the method is called multicolor white LEDs (sometimes known as RGB LEDs). Because they require electronic circuitry to control the mixing and diffusion of different colors, and because individual color LEDs have slightly different emission patterns (leading to color variation depending on viewing direction), even if manufactured in a single unit, they are rarely used to produce white light. However, this method has many applications due to the flexibility it presents in producing color mixing[100]​ and, in principle, for offering greater quantum efficiency in the production of white light.

There are several types of multicolor white LEDs: di-, tri- and tetrachromatic white LEDs. Several key factors influence these different realizations, such as color stability, natural color rendering index and luminous efficiency. Frequently, greater luminous efficiency will imply less naturalness of color, thus creating a trade-off between luminous efficiency and naturalness of colors. For example, dichromatic white LEDs have the best luminous efficiency (120 lm/W), but the lowest color rendering capacity. On the other hand, white tetrachromatic LEDs offer excellent color rendering capacity, but are often accompanied by poor luminous efficiency. Trichromatic white LEDs are in an intermediate position, they have good luminous efficiency (> 70 lm/W) and a reasonable capacity for color reproduction.

Phosphor-based LEDs

This method involves coating LEDs of one color (mainly blue InGaN LEDs) with phosphors of different colors to produce white light; The LEDs resulting from the combination are called phosphor-based white LEDs or phosphor converter white LEDs (PCLED). A fraction of blue light undergoes the Stokes shift that transforms shorter wavelengths into longer wavelengths. Depending on the color of the original LED, phosphors of different colors can be used. If several layers of phosphors of different colors are applied, the emission spectrum is broadened, effectively increasing the color rendering index (CRI) value of a given LED.

The efficiency losses of phosphor-based LEDs (with fluorescent substances) are due to heat losses generated by the Stokes shift and also to other degradation problems related to said fluorescent substances. Compared to normal LEDs, their luminous efficiencies depend on the spectral distribution of the resulting light output and the original wavelength of the LED itself. For example, the luminous efficiency of a typical yellow YAG phosphor of a white LED is 3 to 5 times the luminous efficiency of the original blue LED, due to the higher sensitivity of the human eye for the yellow color than for the blue color (depending on the luminosity function model). Due to the simplicity of its manufacture, the phosphor (fluorescent material) method remains the most popular for achieving high intensity white LEDs. The design and production of a light source or lamp using a monochromatic emitter with fluorescent phosphor conversion is simpler and cheaper than a complex RGB system, and most high-intensity white LEDs on the market today are manufactured using fluorescence light conversion.

Among the challenges that arise to improve the efficiency of LED-based white light sources is the development of more efficient fluorescent substances (phosphors). As of 2010, the most efficient yellow phosphorus continues to be YAG phosphorus, which has a Stokes shift loss of less than 10%. Internal optical losses due to reabsorption in the LED chip itself and in the LED encapsulation constitute 10% to 30% of the efficiency loss. Currently, in the field of phosphor development, a great effort is dedicated to its optimization in order to achieve greater light production and higher operating temperatures. For example, efficiency can be increased by better encapsulation design or by using a more suitable type of phosphor. The adjustment coating process is usually used in order to be able to regulate the variable thickness of the phosphor.

Some white phosphor LEDs consist of blue InGaN LEDs encapsulated in an epoxy resin coated with a phosphor. Another option is to associate the LED with a separate phosphor, a prefabricated piece of preformed polycarbonate coated with the phosphor material. Separate phosphors provide more diffuse light, which is favorable for many applications. Designs with separate phosphors are also more tolerant of variations in the LED emission spectrum. A very common yellow phosphorus material is cerium-doped yttrium aluminum garnet (Ce 3+:YAG).

Other white LEDs

Another method used to produce experimental white light LEDs without the use of phosphors is based on the epitaxy of zinc selenide (ZnSe) growth on a ZnSe substrate that simultaneously emits blue light from its active region and yellow light from the substrate.

A new way to produce white LEDs is to use "Wafer (electronic)") wafers composed of gallium nitride on silicon from 200 mm silicon wafers. This avoids the costly fabrication of sapphire substrates from wafers of relatively small sizes, i.e. 100 or 150 mm. The sapphire apparatus must be attached to a mirror-like collector to reflect light, which would otherwise be lost. It is predicted that by 2020, 40% of all GaN LEDs will be made on silicon. Manufacturing large sapphire is difficult, while large silicon material is cheap and more abundant. On the other hand, LED manufacturers who switch from sapphire to silicon must make a minimal investment.

Organic LEDs (OLED)

In an organic light-emitting diode (OLED), the electroluminescent material that makes up the emitting layer of the diode is an organic compound. The organic material is conductive due to electronic delocalization of the pi bonds caused by the conjugated system in all or part of the molecule; Consequently, the material functions as an organic semiconductor. Organic materials can be small organic molecules in the crystalline phase, or polymers.

One of the advantages made possible by OLEDs is thin, low-cost displays with a low supply voltage, a wide viewing angle, high contrast and a wide color gamut. Polymer LEDs have the added advantage of enabling printable and flexible displays. OLEDs have been used in the manufacture of visual displays for portable electronic devices such as mobile phones, digital cameras and MP3 players, and possible future uses are also considered to include lighting and television.

Quantum dot LEDs

At the beginning of the 60s, a decade of technological revolution began with the birth of the Internet and the discovery of LEDs in the visible spectrum. In 1959, the Nobel Prize in Physics Richard P. Feynman, in his famous lecture given at the annual meeting of the Physical Association of the United States titled: "There is a lot of room at the bottom: an invitation to enter a new field of physics", already anticipated the technological revolution and the important discoveries that could involve the manipulation of materials until they were reduced to atomic or molecular sizes or scales.[102] But it was not until the following decade of the 1970s that knowledge of numerous applications became known. of quantum mechanics (about 70 years after its invention) together with the advancement of materials growth and synthesis techniques, represent an important change in the lines of research of numerous groups.[103]​.

Already in this decade, the ability to design structures having new optical and electronic properties was combined with the search for new technological applications to materials already existing in nature. In fact, in 1969, L. Esaki et al. proposed the implementation of heterostructures formed by very thin layers of different materials, giving rise to what is known as engineering and design of energy bands in semiconductor materials.[104]​ The most basic small-dimensional heterostructure is the quantum well (Quantum Well, QW). It consists of a thin layer of a certain semiconductor, of the order of 100 Å, confined between two layers of another semiconductor material characterized by a greater width of the forbidden energy band (bandgap, BG). Due to the small dimensions of the potential well associated with this structure, the carriers are restricted in their movement to a plane perpendicular to the direction of growth. Laser diodes with QWs in the active zone had great advantages, such as the ability to select the emission wavelength based on the width of the well or the decrease in the threshold current, the latter related to the density of states resulting from confinement in a plane.[105]​.

All these advances were followed naturally by others such as the study of systems with confinement in three dimensions, that is, quantum dots (QDs). Thus, QDs can be defined as artificial systems of very small size, from a few tens of nanometers to a few microns in which the carriers are confined in the three directions of three-dimensional space (that is why it is called zero-dimensional), in a region of space smaller than their Broglie wavelength.

When the size of the semiconductor material that constitutes the quantum dot is within the nanometer scale, this material presents a behavior that differs from that observed for it on a macroscopic scale or for the individual atoms that make it up. The electrons in the nanomaterial are restricted to moving in a very small region of space and are said to be confined. When this region is so small that it is comparable to the wavelength associated with the electron (the de Broglie length), then what is called quantum behavior begins to be observed. In these systems, their physical properties are not explained with classical concepts, but are explained through the concepts of quantum mechanics.[106] For example, the minimum potential energy of an electron confined within a nanoparticle is greater than that expected in classical physics and the energy levels of its different electronic states are discrete. Due to quantum confinement, the size of the particle has a fundamental effect on the density of electronic states and therefore, on its optical response. Quantum confinement occurs when the size of the particles has been reduced until it approaches the radius of the Bohr exciton (generating an electron-hole pair or exciton in the semiconductor material), leaving it confined in a very small space. As a consequence, the structure of the energy levels and the optical and electrical properties of the material are considerably modified. The energy levels become discrete and finite, and depend strongly on the size of the nanoparticle.[106]​.

Los ledes se fabrican en una gran variedad de formas y tamaños. El color de la lente de plástico suele coincidir con el de la luz emitida por el led, aunque no siempre es así. Por ejemplo, el plástico de color púrpura se emplea para los ledes infrarrojos y la mayoría de los ledes azules presentan encapsulamientos incoloros. Los ledes modernos de alta potencia como los empleados para iluminación directa o para retroiluminación aparecen normalmente en montajes de tecnología de superficie") (SMT).

Miniature

Miniature LEDs are often used as indicators. In through-hole technology and surface mounts, their size varies from 2 mm to 8 mm. They normally do not have an independent heat sink.[116]​ The maximum current is between 1 mA and 20 mA. Its small size constitutes a limitation in terms of the power consumed due to its high power density and the absence of a heatsink. They are often connected in a daisy chain to form LED light strips.

The most typical plastic cover shapes are round, flat, triangular and square with a flat top. The encapsulation can also be transparent or colored in order to improve contrast and viewing angles.[117]​.

Researchers at the University of Washington have invented the thinnest LED. It is made up of two-dimensional (2-D) materials. Its width is 3 atoms, that is, between 10 and 20 times thinner than three-dimensional (3-D) LEDs and 10,000 times thinner than a human hair. These 2-D LEDs will enable optical communications and smaller, more energy-efficient nano lasers.[118]​.

There are three main categories of single color miniature LEDs:.

Prepared for a current of 2 mA with about 2 V (consumption of more or less 4 mW).

For a current of 20 mA and with 2 or 4-5 V, designed to be seen in direct sunlight. The 5V and 12V LEDs are normal miniature LEDs that incorporate a series resistor for direct connection to a 5 or 12V supply.

High Power

See also: Solid state lighting, LED Lamp, High Power LEDs or HP-LED").

High-power LEDs (HP-LEDs) or high-emission HO-LEDs (High-Output LEDs) can be controlled with currents from hundreds of mA to more than 1 ampere, while other LEDs only reach tens of milliamps. Some can emit more than a thousand lumens.[119]​[120]​.

Power densities of up to 300 W/(cm²) have also been achieved.[121] As overheating of the LEDs can destroy them, they have to be mounted on a heatsink. If the heat from an HP-LED was not transferred to the medium, the device would fail within a few seconds. An HP-LED can replace an incandescent bulb in a flashlight or several of them can be combined to constitute a power LED lamp. Some well-known HP-LEDs in this category They are those of the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon and Cree X-Lamp Since September 2009, there are LEDs manufactured by Cree that exceed 105 lm/W.[122]​.

Examples of Haitz's law, which predicts an exponential increase over time in the light output and efficiency of an LED, are the CREE XP-GE series that reached 105 lm/W in 2009[122]​ and the Nichia 19 series with an average efficiency of 140 lm/W that was launched in 2010.[123]​.

Powered by alternating current

Seoul Semiconductor has developed LEDs that can run on alternating current without the need for a DC converter. In one half cycle, one part of the LED emits light and the other part is dark, and this happens in reverse during the next half cycle. The normal efficiency of this type of HP-LED is 40 lm/W.[124]​ A large number of LED elements in series can operate directly with the mains voltage. In 2009, Seoul Semiconductor launched a high-voltage LED, called 'Acrich MJT', capable of being driven by AC through a simple control circuit. The low power dissipated by these LEDs provides them with greater flexibility than other original AC LED designs.[125]​.

Applications. Variants

The flashing LEDs are used as attention indicators without the need for any type of external electronics. The flashing LEDs look like standard LEDs, but contain an integrated multivibrator circuit that causes the LEDs to flash with a characteristic period of one second. In LEDs fitted with a diffusion lens, this circuit is visible (a small black dot). Most flashing LEDs emit light in a single color, but more sophisticated devices can flash multiple colors and even fade in a sequence of colors from RGB color mixing.

Bi-color LEDs contain two different LEDs in a single assembly. There are two types; The first consists of two dies connected to two conductors parallel to each other with the current circulating in opposite directions. With current flow in one direction, one color is emitted and with current in the opposite direction, the other color is emitted. In the second type, however, the two dies have separate terminals and there is one terminal for each cathode or each anode, so that they can be controlled independently. The most common color combination is traditional red/green, however, there are other combinations available such as traditional green/amber, red/pure green, red/blue or blue/pure green.

Tri-color LEDs contain three different emitting LEDs in a single frame. Each emitter is connected to a separate terminal so that it can be controlled independently of the others. A very characteristic arrangement is in which four terminals appear, a common terminal (the three anodes or the three cathodes joined together) plus an additional terminal for each color.

RGB LEDs are tri-color LEDs with red, green and blue emitters, generally using a four-wire connection and a common terminal (anode or cathode). This type of LEDs can have both the positive terminal and the negative terminal as common. Other models, however, only have two terminals (positive and negative) and a small built-in electronic control unit.

This type of LED has emitters of different colors and is equipped with only two output terminals. The colors are switched internally by varying the supply voltage.

Alphanumeric LEDs are available as seven-segment displays, fourteen-segment displays, or dot matrix displays. Seven-segment displays can represent all numbers and a limited set of letters while fourteen-segment displays can display all letters. Dot matrix displays typically use 5x7 pixels per character. The use of seven-segment LEDs became widespread in the 1970s and 1980s, but the increasing use of liquid crystal displays has reduced the popularity of numeric and alphanumeric LEDs due to their lower power requirements and greater display flexibility.

They are RGB LEDs that contain their own "smart" control electronics. In addition to power and ground, they have connections for data input and output, and sometimes for clock or strobe signals. They are connected in a daisy chain, with the data input to the first LED equipped with a microprocessor that can control the brightness and color of each of them, independently of the others. They are used where a combination is necessary that provides maximum control and a minimum view of the electronics, such as in Christmas light chains or LED matrices. Some even feature refresh rates in the kHz range, making them suitable for basic video applications.

Power supplies

Main article: Circuit with LED.

The current-voltage characteristic curve of an LED is similar to that of other diodes, in which the current intensity (or briefly, current) grows exponentially with the voltage "Voltage (electricity)") (see Shockley's equation). This means that a small change in voltage can cause a large change in current.[129]​ If the applied voltage exceeds the forward bias threshold voltage drop of the LED, by a small amount, the current limit that the diode can withstand can be greatly exceeded, potentially damaging or destroying the LED. The solution that can be adopted to avoid this is to use constant current intensity power sources (briefly, constant current source[130]​) capable of keeping the current below the maximum value of the current that the LED can pass through or, at least, if a conventional constant voltage source "Voltage (electricity)") or battery is used, add a limiting resistor in series with the LED to the LED lighting circuit. Since normal power sources (batteries, mains) are normally constant voltage sources, most LED fixtures must include a power converter or at least a current limiting resistor. However, the high resistance of three-volt button cells combined with the high differential resistance of nitride-derived LEDs makes it possible to power such LEDs with a button cell without the need to incorporate an external resistor.

electrical polarity

Main article: Electrical polarity of LEDs.

As with all diodes, current flows easily from the p-type material to the n-type material.[131] However, if a small voltage is applied in the reverse direction, current does not flow and no light is emitted. If the reverse voltage rises enough to exceed the breakdown voltage, a high current flows and the LED may be damaged. If the reverse current is limited enough to prevent damage, the reverse driving LED can be used as an avalanche diode.

Health and safety

The vast majority of devices containing LEDs are "safe under normal use", and are therefore classified as "Risk Product 1 RG1 (low risk)" / "LED Class 1". Currently, only a few LEDs—extremely bright LEDs that have a very small viewing angle of an aperture of 8° or less—could, in theory, cause temporary blindness and are therefore classified as "Risk 2 RG2 (moderate risk)."[132]​ LED technology is also used in the healthcare sector to reduce energy consumption costs and slow the spread of hospital-acquired infections. Its use is also being studied for the treatment of pain, insomnia and other disorders and diseases, among others, Alzheimer's.[133]​.

The opinion of the French Agency for Food, Environmental and Occupational Health and Safety (ANSES), when addressing health issues related to LEDs in 2010, suggested prohibiting the public use of lamps that were in Group 2 or Moderate Risk, especially those with a high blue component, in places frequented by children.[134]​.

In general, safety regulations for the use of laser light[135]​[136]​—and Risk 1, Risk 2 devices, etc.—are also applicable to LEDs.[137]​.

Just as LEDs have the advantage, over fluorescent lamps, that they do not contain mercury "Mercury (element)"), however, they may contain other dangerous metals such as lead and arsenic. Regarding the toxicity of LEDs when treated as waste, a study published in 2011 stated: "According to federal regulations, LEDs are not hazardous, except for low-intensity red LEDs, because they initially contained Pb (lead) in concentrations above the regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), lead (up to 8103 mg/kg, limit: 1000), nickel (up to 4797 mg/kg, limit: 2000), or silver (up to 721 mg/kg, limit: 500) cause all LEDs, except low intensity yellow ones, to be dangerous."[138]

Indicators and signal lamps

The low energy consumption, low need for maintenance and small size of LEDs has led to their use as status and display indicators in a wide variety of equipment and installations. Large-area LED screens are used to broadcast the game in stadiums, as dynamic decorative screens and as dynamic message signs on highways. Light, thin message displays are used in airports and railway stations and as destination information panels on trains, buses, trams and ferries.

Single-color lights are suitable for traffic lights, traffic signs, "Emergency Exit (Safety)" exit signs, vehicle emergency lighting, navigation lights, headlights (the standard chromaticity and luminance indices were established in the International Convention for the Prevention of Collisions at Sea 1972 Annex 1 and by the International Commission on Illumination or CIE) and Christmas lights composed of LEDs. In regions with cold climates, LED traffic lights may remain covered in snow.[164] Red or yellow LEDs are used in indicators and alphanumeric displays, in environments where night vision must be maintained: airplane cockpits, underwater and ship bridges, astronomical observatories and in the field, for example for the observation of animals at night and military field applications.

Given their long lifespan, fast switching times, and ability to be seen in broad daylight due to their high intensity and concentration, LEDs have been used for some time for brake lights in cars, trucks, and buses, and in direction change signals; Many vehicles currently use LEDs in their rear light assemblies. The use in brakes improves safety due to the great reduction in the time required for a complete ignition, that is, due to the fact that it has a shorter rise time, up to 0.5 seconds faster than an incandescent bulb. This provides more reaction time for the drivers behind. In a two-intensity circuit (rear marker lights and brake lights) if the LEDs are not actuated at a fast enough frequency, they can create a ghost array, where LED ghost images will appear if the eyes move quickly across the light array. Headlights with white LEDs are beginning to be used. The use of LEDs has stylistic advantages because they can form much thinner beams of light than incandescent lamps fitted with parabolic reflectors.

Low-power LEDs are relatively very economical and allow their use in short-lived luminous objects such as luminous self-adhesives, disposable objects and the Lumalive photonic fabric. Artists also use LEDs for so-called LED art. Weather and distress radio receivers with coded area messages (SAME) have three LEDs: red for alarms, orange for attention and yellow for warnings, indications and reports.

Lightning

To encourage the switch to LED lamps, the United States Department of Energy has created the L Prize. The Philips Lighting North America LED bulb won the first prize on August 3, 2011 after successfully completing 18 months of intensive field, laboratory and product testing.[165]​.

LEDs are used as street lights and in architectural lighting. Mechanical robustness and long lifespan are used in automotive lighting in cars, motorcycles and bicycle lights. LED light emission can be effectively controlled by using non-imaging optical principles.

In 2007, the Italian town of Torraca was the first place to convert its entire lighting system to LEDs.[166] LEDs are also used in aviation, Airbus has used LED lighting on its Airbus A320 since 2007, and Boeing uses LED lighting on the 787. LEDs are also now used in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline lights, and edge lights.

LEDs are also used as a light source for DLP projectors and to illuminate LCD televisions (known as LED televisions) and laptop screens. RGB LEDs increase the color range by up to 45%. TV screens and computer screens can be made thinner by using LEDs for backlighting.[167]​ The lack of infrared or thermal radiation makes LEDs ideal for stage lighting with banks of RGB LEDs that can easily change color and decrease lighting warm-up, as well as medical lighting where IR radiation can be harmful. In energy conservation, there is less heat production when using LEDs.

They are also small, durable and require little power, which is why they are used in portable devices such as flashlights. LED strobes or "Flash (photography)" camera flashes operate at a safe, low voltage, rather than the 250+ volts commonly found in xenon flash based lighting. This is especially useful in cell phone cameras. LEDs are used for infrared illumination in night vision applications including security cameras. A ring of LEDs around a forward-facing video camera on a retroreflective background enables chroma keying in video productions.

LEDs are used in mining operations, as cap lamps to provide light to miners. Research has been conducted to improve mining LEDs, reduce glare and increase illumination, reducing the risk of injury to miners.[168]​.

LEDs are now commonly used in all market areas, from commercial to domestic use: standard lighting, theatrical, architectural, public installations, and wherever artificial light is used.

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement, and new technologies such as AmBX, exploiting the versatility of LEDs. NASA has even sponsored research into the use of LEDs to promote health for astronauts.[169]​.

Optical communications. Data transfer and other communications

Light can be used to transmit data and analog signals. For example, white LEDs can be used in systems to help people orient themselves in closed spaces in order to locate arrangements or objects.[170]​.

"Assisted listening devices" in many theaters and similar spaces use arrays of infrared LEDs to send sound to spectators' receivers. LEDs (and also semiconductor lasers) are used to send data over many types of fiber optic cable. From TOSLINK cables for digital audio transmission to the very high-bandwidth fiber links that form the backbone of the Internet. For a time, computers were equipped with IrDA interfaces, allowing them to send and receive data of nearby equipment using infrared radiation.

Because LEDs can turn on and off millions of times per second, they require very high bandwidth for data transmission.[171]​[172]​.

Sustainable lighting

Lighting efficiency is necessary for sustainable architecture. In 2009, tests carried out with LED bulbs by the United States Department of Energy showed an average efficiency from 35 lm/W, therefore below the efficiency of CFLs, to values ​​as low as 9 lm/W, worse than incandescent bulbs. A typical 13-watt LED bulb emitted 450 to 650 lumens,[173]​ which was equivalent to a standard 40-watt incandescent bulb.

In any case, in 2011 there were LED bulbs with an efficiency of 150 lm/W, and even low-end models exceeded 50 lm/W, so a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The latter have a durability of 1,000 hours while an LED can continue operating at a lower efficiency for more than 50,000 hours.[174]​.

Comparative table of led-LFC-incandescent bulb:.

The reduction in electrical energy consumption achieved with LED-based lighting is important when compared to incandescent lighting. Furthermore, this reduction also manifests itself as a notable decrease in damage to the environment. Each country presents a different energy panorama and, therefore, although the impact on energy consumption is the same, the production of gases harmful to the environment may fluctuate somewhat from one country to another. Regarding consumption, a conventional 40-watt incandescent bulb can be taken as an example. An equivalent light output can be obtained with a 6 watt LED system. By using the LED system instead of incandescent bulbs, energy consumption can be reduced by more than 85%. Regarding the savings in environmental impact, it is possible to quantify it for any country if the CO production for each kW per hour is known. In the specific case of Spain, it is known that the energy mix of the Spanish electricity grid has produced about 308 g of CO/kWh in 2016. It is assumed for the calculation that both the bulb and the LED assembly have operated for 10 hours a day throughout the year 2016.[175]​ The energies consumed have been 146 kW-hour by the incandescent bulb and 21.6 kW-hour by part of the led set. The electrical energy consumed can be translated into kg of CO produced per year. In the first case, the generation of about 45 kg of CO has been carried out, while in the second case the production of CO has been reduced to 6.75 kg.

Light sources for artificial vision systems

Industrial vision systems usually require homogeneous lighting to be able to focus on image features of interest. This is one of the most frequent uses of LED lights, and it will surely continue to do so, driving down the prices of systems based on light signaling. Barcode scanners are the most common example of vision systems; many of these low-cost products use LEDs instead of lasers.[176] Optical computer mice also use LEDs for their vision system, as they provide a uniform light source over the surface for the miniature camera inside the mouse. In fact, LEDs are an almost ideal light source for vision systems for the following reasons:.

• - The size of the illuminated field is usually comparatively small and machine vision systems are often quite expensive, so the cost of the light source is usually less of a concern. However, it may not be easy to replace a broken light source within complex machinery; In this case the long lifespan of the LEDs is a benefit.

• - LED components tend to be small and can be placed at high density on flat or uniform surface substrates (PCB, etc.) so that homogeneous light sources can be designed that direct light from strictly controlled directions onto inspected parts. This can often be achieved with small, low-cost lenses and diffusers, helping to achieve high light densities with control over illumination levels and homogeneity. LED sources can be configured in various ways (spotlights for reflective lighting, ring lights for coaxial lighting, backlights for contour lighting, linear mounts, large format flat panels, dome sources for diffuse omnidirectional lighting).

• - LEDs can be easily stroboscopic (in the microsecond range and below) and image synchronized. High power LEDs are available to allow well-illuminated images, even with very short light pulses. This is often used to get sharp, sharp images of fast-moving parts.

• - LEDs come in various colors and wavelengths, allowing easy use of the best color for each need, where the different color can provide better visibility of features of interest. Having a precisely known spectrum allows closely matched filters to be used to separate the informational bandwidth or to reduce the disturbing effects of ambient light. LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation. This allows the use of plastic lenses, filters and diffusers. Waterproof units can also be easily designed, allowing use in harsh or humid environments (food, beverage, oil industries).

Medicine and biology

Healthcare has echoed the advantages of LEDs over other types of lighting and has incorporated them into its latest generation equipment. The advantages offered by LEDs in their current state of development have led to their rapid diffusion in the world of instruments for diagnosis and support in medical and surgical procedures. The advantages appreciated by medical professionals are the following:.

• - The small size of the light sources that, in general, can be associated with very thin and flexible light guides, which allows them to move inside thin catheters.

• - The lack of accompanying infrared radiation, which allows the adjective cold light to be associated with them. The heat given off by other types of light sources made their use difficult or impossible in certain diagnostic observations or surgical interventions.

• - The white tone that is usually the favorite for medical observations. It must be a natural white color capable of presenting all colors without metamerism problems. The natural color of the tissues illuminated in this way favors the correct diagnosis of the observed field.

• - The high luminous intensity achievable by these light sources.

Based on the previous ideas, current endoscopes are equipped with LED lighting. The endoscopic technique covers many medical specialties, for example gastroscopy, colonoscopy, laryngoscopy, otoscopy or arthroscopy. All of these techniques allow the observation of organs and systems of the human body through the use of miniature video cameras. They can also be used in surgical interventions or to make diagnoses. The equipment is also known as videoscopes or videoendoscopes. There are rigid or flexible ones depending on the needs. Fiber optics adapt to each particular case. On the other hand, the lighting in operating rooms and dental clinics are currently LED. They perfectly satisfy all the technical and health requirements for their use. Particularly appreciated is the obtaining of bright, natural, white lighting (more than one hundred and fifty thousand candelas one meter away from the field of operation), without shadows and without infrared or ultraviolet emissions that could affect both the patient and the medical staff participating in the intervention.

The same thing happens with the headlamps of surgeons and dentists equipped with LEDs, with lamps for medical examinations, for ophthalmological examinations and interventions or for minor surgery, so it can be said that LEDs have come to cover all medical specialties. Optical companies dedicated to medicine have incorporated LEDs into their observation equipment, for example in microscopes, thereby obtaining many advantages for the study of images using different techniques (bright field, contrast, fluorescence), which is evident in the advertising and commercial fields. LEDs are successfully used as sensors in heart rate monitors or oxygen blood pressure monitors to measure oxygen saturation.

Industry

The industry has adapted the observation models used in medicine for its own needs and the equipment is called "industrial endoscopes" or also borescopes), flexoscopes) or video endoscopes). With them you can observe the interior of machines, engines, ducts, cavities or weapons without having to dismantle them.

Other applications

The light from LEDs can be modulated very quickly, which is why they are widely used in fiber optics and free space optical communication. This includes remote controls used in LED televisions, VCRs and computers. Optical isolators use an LED combined with a photodiode or phototransistor to provide an electrically isolated signal path between two circuits. This is especially useful in medical equipment where the signals from a low voltage sensor circuit (typically battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits that do not share a common ground potential.

Many sensor systems rely on light as a signal source. LEDs are often ideal as a light source due to sensor requirements. LEDs are used as motion sensors"), for example in optical computer mice "Mouse (computing)"). The Nintendo Wii Sensor Bar uses infrared LEDs. Pulse oximeters use them to measure oxygen saturation. Some tabletop scanners use RGB LED arrays instead of the typical cold cathode fluorescent lamp as a light source. Having independent control of three illuminated colors allows the scanner to be calibrated for a more accurate color balance and not There is no need for heating. In addition, their sensors only need to be monochromatic, since at any time the scanned page is only illuminated with one color of light. Since LEDs can also be used as photodiodes, they can also be used for photo emission or detection. This could be used, for example, in a touch screen that records light reflected from a finger or a stylus. photosynthesis in plants,[178]​ and bacteria and viruses can be eliminated from water and other substances using UV LEDs for sterilization.

LEDs have also been used as a quality voltage reference in electronic circuits. Instead of a Zener diode in low voltage regulators, forward voltage drop can be used (for example, about 1.7 V for a normal red LED). Red LEDs have the flattest I/V curve. Although the LED forward voltage is much more current dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available.

The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable for incorporation into thin materials, has encouraged experimentation in combining light sources and interior wall covering surfaces.[179]​ The new possibilities offered by these developments have led some designers and companies, such as Meystyle"),[180]​ Ingo Maurer,[181]​ Lomox,[182]​ and Philips[183]​ to research and develop proprietary LED wallpaper technologies, some of which are currently available for commercial purchase. Other solutions exist primarily as prototypes or are in the process of being refined.

• - OLED.

• - Plasma screen.

• - Wikimedia Commons hosts a multimedia category on led.

• - Wikimedia Commons hosts a multimedia category on led (SMD) "commons:Category:Light-emitting diodes (SMD)").

• - Wiktionary has definitions and other information about LED.

• - on YouTube.

Discovery and first devices

The phenomenon of electroluminescence was discovered in 1907 by British experimenter Henry Joseph Round of the Marconi Laboratories, using a silicon carbide crystal and a cat's whisker detector. Soviet inventor Oleg Losev reported the construction of the first LED in 1927. His research appeared in Soviet, German, and British scientific journals, but the discovery was not put into practice until several decades later. Kurt Lehovec, Carl Accardo and Edward Jamgochian interpreted the mechanism of these first LED diodes in 1951, using an apparatus that used silicon carbide crystals, with a pulse generator and a power supply, and in 1953 with a pure variant of the crystal.

Rubin Braunstein of RCA reported in 1955 on the infrared emission from gallium arsenide (GaAs) and other semiconductor alloys. Braunstein observed that this emission was generated in diodes constructed from alloys of gallium antimonide (GaSb), gallium arsenide (GaAs), indium phosphide (InP) and silicon-germanium (SiGe) at room temperature and 77 kelvin.

In 1957, Braunstein also demonstrated that these rudimentary devices could be used to establish non-radio communication over short distances. As Kroemer points out, Braunstein established a very simple line of optical communications:[18]​ he took music from a record player and processed it using the appropriate electronics to modulate the direct current produced by a GaAs gallium arsenide diode. The light emitted by the GaAS diode was able to sensitize a PbS lead sulfide "Lead(II) sulfide") diode located at a certain distance. The signal thus generated by the PbS diode was introduced into an audio amplifier and transmitted through a speaker. When the light beam was intercepted between the two LEDs, the music stopped. This assembly already foreshadowed the use of LEDs for optical communications.

In September 1961, James R. Biard and Gary Pittman, working at Texas Instruments (TI) in Dallas, Texas, discovered infrared radiation (900 nm) from a tunnel diode they had constructed using a gallium arsenide (GaAs) substrate.[19] In October 1961, they demonstrated efficient light emissions and signal coupling between the arsenide p-n junction. of light-emitting gallium and an electrically insulated photodetector constructed from a semiconductor material.[20]​ Based on their discoveries, on August 8, 1962, Biard and Pittman produced a patent titled “Semiconductor Radiant Diode”[21]​ that described how a zinc alloy diffused during crystal growth that forms the substrate of a p-n LED junction with a sufficiently spaced cathode contact, allowed Efficient infrared light emission in direct polarization.

In view of the importance of their research, as it appeared in their engineering notebooks and even before communicating their results from the laboratories of General Electric, Radio Corporation of America, IBM, Bell Laboratories or those of the Lincoln Laboratory of the Massachusetts Institute of Technology, the United States Patent and Trademark Office granted them a patent for the invention of gallium arsenide infrared light-emitting diodes (patent US3293513A of the USA),[22]​ which are considered the first LEDs for practical use. Immediately after the patent was filed, TI began a project to manufacture the infrared diodes. In October 1962, Texas Instruments developed the first commercial LED (the SNX-100), which used a pure gallium arsenide crystal to emit 890 nm light. In October 1963, TI launched the first commercial hemispherical LED, the SNX-110.[23]​.

The first LED with emission in the visible (red) spectrum was developed in 1962 by Nick Holonyak Jr. when he worked at General Electric. Holonyak reported in the journal Applied Physics Letters on December 1, 1962. telecommunications through optical fibers. To this end, he discovered new semiconductor materials expressly adapted to the wavelengths of the aforementioned transmission through optical fibers.[26]​.

Initial business development

The first commercial LEDs were generally used to replace incandescent lamps and neon indicator lamps as well as in seven-segment displays.[27] First in expensive equipment such as electronic and laboratory test equipment, and later in other electrical devices such as televisions, radios, telephones, calculators, as well as wristwatches. Until 1968, visible and infrared LEDs were extremely expensive, on the order of $200 per unit, so they had little practical use.[28]​ The Monsanto Company was the first to mass produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs for indicators.[28]​.

Hewlett-Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. These red LEDs were bright enough to be used as indicators, since the light emitted was not enough to illuminate an area. The readings on the calculators were so weak that plastic lenses were placed over each digit to make them legible. Later, other colors appeared and were widely used in gadgets and equipment. In the 1970s Fairchild Optoelectronics manufactured commercially successful LED devices for less than five cents each. These devices used compound semiconductor chips manufactured using the planar process invented by Jean Hoerni of Fairchild Semiconductor.[29]​[30]​ Planar processing for chip manufacturing combined with innovative encapsulation methods allowed the team led by optoelectronics pioneer Thomas Brandt to achieve the cost reductions needed at Fairchild.[31]​ These methods continue to be used by LED manufacturers.[32]​.

Most LEDs were manufactured in the typical 5mm T1¾ and 3mm T1 packages, but with increasing power output, it has become increasingly necessary to remove excess heat to maintain reliability.[33]​ It has therefore been necessary to design more complex packages designed to achieve efficient heat dissipation. The encapsulations currently used for high-power LEDs bear little resemblance to those of the first LEDs.

blue led

Blue LEDs were first developed by Henry Paul Maruska of RCA in 1972 using Gallium Nitride (GaN) on a sapphire substrate.[34]​[35]​

SiC type LEDs (made with silicon carbide) began to be marketed by Cree, Inc., United States in 1989.[36] However, none of these blue LEDs were very bright.

The first high-brightness blue LED was presented by Shuji Nakamura of Nichia Corp. in 1994, starting from the material Indium-Gallium Nitride (InGaN). For their research, Nakamura, Akasaki and Amano were awarded the Nobel Prize in Physics.[39][40] In 1995, Alberto Barbieri from the Cardiff University laboratory (UK) investigated the efficiency and reliability of high-brightness LEDs and as a result of the research obtained an LED with a transparent contact electrode using indium tin oxide (ITO) on aluminum-gallium-indium phosphide and gallium arsenide.

In 2001[41]​ and 2002[42]​ processes were carried out to grow gallium nitride LEDs on silicon. As a consequence of these investigations, in January 2012 Osram launched high-power gallium-indium nitride LEDs grown on a silicon substrate.[43]​.

White LED and evolution

The achievement of high efficiency in blue LEDs was quickly followed by the development of the first white LED. In such a device a Y Al O:Ce coating “phosphor” (fluorescent material) (known as YAG or yttrium aluminum garnet) absorbs some of the blue emission and generates yellow light by fluorescence. In a similar way, it is possible to introduce other “matches” that generate green or red light by fluorescence. The resulting mixture of red, green and blue is perceived by the human eye as white; On the other hand, it would not be possible to appreciate red or green objects by illuminating them with the YAG phosphor since it generates only yellow light along with a remnant of blue light.

The first white LEDs were expensive and inefficient. However, the intensity of light produced by LEDs has increased exponentially, with a doubling time occurring approximately every 36 months since the 1960s (according to Moore's Law). This trend is generally attributed to a parallel development of other semiconductor technologies and advances in optics and materials science, and has been named Haitz's law in honor of Roland Haitz.[44]

The light output and efficiency of blue and near-ultraviolet LEDs increased while the cost of lighting devices manufactured with them decreased, which led to the use of white light LEDs for lighting. The fact is that they are replacing incandescent and fluorescent lighting.[45][46]​.

White LEDs can produce 300 lumens per electrical watt while lasting up to 100,000 hours. Compared to incandescent bulbs, this represents not only a huge increase in electrical efficiency, but also a similar or lower cost per bulb.[47]​.

A P-N junction can provide an electric current when illuminated. Similarly, a P-N junction crossed by a direct current can emit light photons. There are two ways of considering the phenomenon of electroluminescence. In the second case, this could be defined as the emission of light by a semiconductor when it is subjected to an electric field. The charge carriers recombine in a forward-biased P-N junction. Specifically, the electrons in the N region cross the potential barrier and recombine with the holes in the P region. The free electrons are in the conduction band while the holes are in the valence band. In this way, the energy level of holes is lower than that of electrons. When electrons and holes recombine, a fraction of the energy is emitted in the form of heat and another fraction in the form of light.

The physical phenomenon that takes place in a PN junction when the current passes in forward polarization, therefore, consists of a succession of electron-hole recombinations. The phenomenon of recombination is accompanied by the emission of energy. In ordinary Germanium or Silicon diodes, phonons or vibrations of the crystalline structure of the semiconductor are produced that simply contribute to its heating. In the case of LED diodes, the semiconductor materials are different from the previous ones, for example, being various type III-V alloys such as gallium arsenide (AsGa), gallium phosphide") (PGa) or gallium phosphoarsenide (PAsGa).

In these semiconductors, the recombinations that develop in the PN junctions eliminate excess energy by emitting light photons. The color of the emitted light depends directly on its wavelength and is characteristic of each specific alloy. Currently, alloys are manufactured that produce luminous photons with wavelengths in a wide range of the electromagnetic spectrum within the visible, near-infrared and near-ultraviolet. What is achieved with these materials is to modify the energy width of the bandgap, thus modifying the wavelength of the emitted photon. If the LED diode is reverse polarized, the recombination phenomenon will not occur, so it will not emit light. Reverse bias can damage the diode.

The electrical behavior of the LED diode in forward polarization is as follows. If the polarization voltage is increased, from a certain value (which depends on the type of semiconductor material), the LED begins to emit photons, the turn-on voltage has been reached. Electrons can move across the junction by applying different voltages to the electrodes; Thus, the emission of photons begins and as the polarization voltage increases, the intensity of the emitted light increases. This increase in luminous intensity is coupled with the increase in current intensity and can be decreased by Auger recombination. During the recombination process, the electron jumps from the conduction band to the valence band, emitting a photon and accessing, by conservation of energy and momentum, a lower energy level, below the Fermi level of the material. The emission process is called radiative recombination, which corresponds to the phenomenon of spontaneous emission. Thus, in each electron-hole radiative recombination, a photon of energy equal to the width in energies of the bandgap is emitted:.

where c is the speed of light and f and λ are the frequency and wavelength, respectively, of the light it emits. This description of the basis of the emission of electromagnetic radiation by the LED diode can be seen in the figure where a schematic representation of the PN junction of the semiconductor material is made together with the energy diagram, involved in the recombination process and light emission, in the lower part of the drawing. The wavelength of the emitted light, and therefore its color, depends on the width of the energy bandgap. The most important substrates available for application in light emission are GaAs and InP. LED diodes can reduce their efficiency if their absorption and spectral emission peaks depending on their wavelength are very close, as is the case with GaAs:Zn (zinc-doped gallium arsenide) LEDs, since part of the light they emit is absorbed internally.

The materials used for LEDs have a band gap in direct polarization whose width in energies varies from infrared light to visible light or even near ultraviolet light. The evolution of LEDs began with infrared and red gallium arsenide devices. Advances in materials science have made it possible to manufacture devices with increasingly shorter wavelengths, emitting light in a wide range of colors. LEDs are generally fabricated on an N-type substrate, with an electrode connected to the P-type layer deposited on its surface. P-type substrates, although less common, are also manufactured.

Physical foundation

An LED begins to emit when a voltage of 2-3 volts is applied to it. In reverse polarization, a different vertical axis is used than in forward polarization to show that the current absorbed is practically constant with the voltage until breakdown occurs.

The LED is a diode made up of a semiconductor chip doped "Doping (semiconductors)") with impurities that create a PN junction. As in other diodes, current flows easily from the p side, or anode, to the n side, or cathode, but not in the opposite direction. Charge carriers (electrons and holes) flow into the junction from two electrodes set at different voltages (voltage (electricity)). When an electron recombines with a hole, its energy level drops and the excess energy is released in the form of a photon. The wavelength of the emitted light, and therefore the color of the LED, depends on the energy width of the bandgap corresponding to the materials that make up the pn junction.

In silicon or germanium diodes, electrons and holes recombine, generating a non-radiative transition, which does not produce any light emission, since they are semiconductor materials with an indirect bandgap. The materials used in LEDs have a direct bandgap with an energy width that corresponds to the light spectrum of the near-infrared (800-2500 nm), the visible and the near-ultraviolet (200-400 nm).

The development of LEDs began with red and infrared light devices, made with gallium arsenide (GaAs). Advances in materials science have made it possible to build devices with increasingly smaller wavelengths, emitting light within a wide range of colors.

LEDs are typically made from an n-type substrate, with one of the electrodes bonded to the p-type layer deposited on its surface. p-type substrates are also used, although they are less common. Many commercial LEDs, especially GaN/InGaN, also use sapphire (aluminum oxide) as a substrate.

Most of the semiconductor materials used in the manufacture of LEDs have a very high refractive index. This implies that the majority of the light emitted inside the semiconductor is reflected when it reaches the outer surface that is in contact with the air by a phenomenon of total internal reflection. The extraction of light constitutes, therefore, a very important aspect and one in constant research and development to take into consideration in the production of LEDs.

refractive index

Most of the semiconductor materials used in the manufacture of LEDs have a very high refractive index with respect to air. This implies that the majority of the light emitted inside the semiconductor will be reflected when it reaches the outer surface that is in contact with the air by a phenomenon of total internal reflection.

This phenomenon affects both the light emission efficiency of the LEDs and the light absorption efficiency of the photovoltaic cells. The refractive index of silicon is 3.96 (at 590 nm),[48]​ while that of air is 1.0002926.[48]​ Light extraction constitutes, therefore, a very important aspect and one in constant research and development to take into consideration in the production of LEDs.

In general, an uncoated flat surface LED semiconductor chip will emit light only in the direction perpendicular to the surface of the semiconductor and in very close directions, forming a cone called light cone[49]​ or escape cone.[50]​ The maximum angle of incidence that allows photons to escape from the semiconductor is known as the critical angle. When this angle is exceeded, the photons no longer escape from the semiconductor, but instead are reflected within the semiconductor crystal as if there were a mirror on the outer surface.[50]​.

Due to internal reflection, light that has been reflected internally on one face can escape through other crystal faces if the angle of incidence now becomes sufficiently low and the crystal is sufficiently transparent not to reflect the photon emission back inward. However, in a simple cubic LED with external surfaces at 90 degrees, all faces act as equal angled mirrors. In this case, most of the light cannot escape and is lost as heat within the semiconductor crystal.[50]​.

A chip that has angled facets on its surface similar to those of a cut jewel or a Fresnel lens can increase light output by allowing its emission in orientations that are perpendicular to the outer facets of the chip, typically more numerous than the only six in a cubic sample.[51]​.

The ideal shape of a semiconductor to obtain the maximum light output would be that of a microsphere") with the emission of the photons located exactly in the center of it, and equipped with electrodes that penetrate to the center to connect with the point of emission. All the light rays that start from the center would be perpendicular to the surface of the sphere, which would result in no internal reflections. A hemispherical semiconductor would also work correctly since the flat part would act as a mirror to reflect the photons so that all light could be emitted completely through the hemisphere.[52]​.

After building a wafer "Wafer (electronic)") of semiconductor material, it is cut into small fragments. Each fragment is called a chip and becomes the small active part of a light-emitting LED diode.

Many LED semiconductor chips are encapsulated or incorporated inside molded plastic housings. The plastic casing aims to achieve three purposes:

1. • Facilitate the assembly of the semiconductor chip in lighting devices.

2. • Protect the fragile electrical wiring associated with the diode from physical damage.

3. • Act as an intermediary element for the refraction effect between the high index of the semiconductor and that of the air.

The third feature contributes to increasing the light emission from the semiconductor by acting as a diffuser lens, allowing light to be emitted to the outside with an angle of incidence on the outer wall much greater than that of the narrow cone of light coming from the uncoated chip.

Efficiency and operational parameters

LEDs are designed to operate with an electrical power no greater than 30-60 milliwatts (mW). Around 1999, Philips Lumileds introduced more powerful LEDs capable of working continuously at a power of one watt. These LEDs used much larger die-cut semiconductors in order to accept higher power supplies. Additionally, they were mounted on metal rods to facilitate heat removal.

One of the main advantages of LED-based lighting sources is the high luminous efficiency. White LEDs quickly matched and even surpassed the efficiency of standard incandescent lighting systems. In 2002, Lumileds manufactured five-watt LEDs, with a luminous efficiency of 18-22 lumens per watt (lm/W). For comparison, a conventional 60-100 watt incandescent bulb emits around 15 lm/W, and standard fluorescent lamps emit up to 100 lm/W.

As of 2012, Future Lighting Solutions") had achieved the following efficiencies for some colors.[53] Efficiency values ​​show the luminous power output per watt of input electrical power. The luminous efficiency values ​​include the characteristics of the human eye and have been deduced from the luminosity function.

In September 2003, Cree Inc." manufactured a new type of blue LED that consumed 24 milliwatts (mW) at 20 milliamps (mA). This enabled a new encapsulation of white light that produced 65 lm/W at 20 milliamps, making it the brightest white LED available on the market; it was also more than four times more efficient than standard incandescent bulbs. In 2006 they introduced a white LED prototype with a record luminous efficiency of 131 lm/W for a current of 20 milliamps. Nichia Corporation") has developed a white LED with a luminous efficiency of 150 lm/W and a direct current of 20 mA.[54] The LEDs from the company Cree Inc. called xlamp xm-L, came onto the market in 2011, producing 100 lm/W at the maximum power of 10 W, and up to 160 lm/W with an input electrical power of about 2 W. In 2012, Cree Inc. introduced a white LED capable of producing 254 lm/W,[55]​ and 303 lm/W in March 2014.[56]​ General lighting needs in practice require high-power LEDs, one watt or more. They operate with currents greater than 350 milliamps.

These efficiencies refer to the light emitted by the diode kept at a low temperature in the laboratory. Since LEDs, once installed, operate at high temperatures and with conduction losses, the efficiency is actually much lower. The United States Department of Energy (DOE) has carried out tests to replace incandescent lamps or CFLs with LED lamps, showing that the average efficiency achieved is about 46 lm/W in 2009 (the behavior during the tests remained within a range of 17 lm/W to 79 lm/W).[57]​.

When the electric current supplied to an LED exceeds a few tens of milliamps, the luminous efficiency decreases due to an effect called loss of efficiency.

At first, an explanation was sought attributing it to high temperatures. However, scientists were able to demonstrate the opposite, that although the life of the LED may be shortened, the drop in efficiency is less severe at elevated temperatures.[58]​ In 2007, the cause of the decrease in efficiency was attributed to Auger recombination") which gives rise to a mixed reaction.[59]​ Finally, a 2013 study definitively confirmed this theory to justify the loss of efficiency.[60]​.

Half-life and failure analysis

Solid state devices such as LEDs have very limited obsolescence if operated at low currents and low temperatures. Life times are 25,000 to 100,000 hours, but the influence of heat and current can increase or decrease this time significantly.[66]​.

The most common failure of LEDs (and laser diodes) is the gradual reduction of light output and loss of efficiency. The first red LEDs stood out for their short life. With the development of high-power LEDs, devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress in the material and can cause early degradation of light output. To quantitatively classify the useful life in a standardized way, it has been suggested to use the L70 or L50 parameters, which represent the life times (expressed in thousands of hours) in which a given LED reaches 70% and 50% of the initial light emission, respectively.[67]​.

Just as in most previous light sources (incandescent lamps, discharge lamps, and those that burn a fuel, for example candles and oil lamps) the light was generated by a thermal process, LEDs only work correctly if they are kept sufficiently cool. The manufacturer usually specifies a maximum junction temperature between 125 and 150°C, and lower temperatures are recommended in the interest of achieving long life for the LEDs. At these temperatures, relatively little heat is lost through radiation, meaning that the light beam generated by an LED is considered cold.

The waste heat in a high-power LED (which as of 2015 can be considered less than half the electrical power it consumes) is transported by conduction through the substrate and encapsulation to a heat sink, which removes the heat into the environment by convection. It is therefore essential to carry out a careful thermal design, taking into account the thermal resistances of the LED encapsulation, the heat sink and the interface between the two. Medium power LEDs are typically designed to be soldered directly to a printed circuit board that has a thermally conductive metal layer. High power LEDs are encapsulated in large surface area ceramic packages designed to be connected to a metal heat sink, the interface being a material with high thermal conductivity (thermal paste, phase change material, conductive thermal pad") or hot melt glue).

If an LED lamp is installed in a lighting fixture without ventilation, or the environment lacks fresh air circulation, the LEDs are likely to overheat, reducing their lifespan or even causing early deterioration of the lighting fixture. Thermal design is typically designed for an ambient temperature of 25°C (77°F). LEDs used in outdoor applications, such as traffic signs or pavement marking lights, and in climates where the temperature inside the lighting fixture is very high, can experience anything from reduced luminous output to complete failure.[68]​.

Contenido

Los ledes convencionales están fabricados a partir de una gran variedad de materiales semiconductores inorgánicos. En la siguiente tabla se muestran los colores disponibles con su margen de longitudes de onda, diferencias de potencial de trabajo y materiales empleados.

Blue and ultraviolet

The first blue-violet LED used magnesium-doped chlorine and was developed by Herb Maruska and Wally Rhines at Stanford University in 1972, PhD students in materials science and engineering.[82][83] At that time Maruska was working in the RCA laboratories, where he collaborated with Jacques Pankove. In 1971, a year after Maruska left for Stanford, his RCA colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc doped with gallium nitride; However, the device Pankove and Miller built, the first true gallium nitride light-emitting diode, emitted green light.[84] In 1974 the US Patent Office granted Maruska, Rhines, and Stanford professor David Stevenson a patent (US patent US3819974 A)[85] for their 1972 work on doping gallium nitride with magnesium that today continues to be the basis of all commercial blue LEDs and laser diodes. These devices built in the 1970s did not have sufficient luminous output for practical use, so research into gallium nitride diodes slowed down. In August 1989, Cree introduced the first commercial blue LED with an indirect transition through the bandgap in a silicon carbide (SiC) semiconductor.[86]​[87]​ SiC LEDs have a very low luminous efficiency, no more than 0.03%, but emit in the visible blue region.

In the late 1980s, breakthroughs in epitaxial growth and p-type doping in GaN ushered in the modern era of GaN opto-electronic devices. Based on the above, Theodore Moustakas patented a method of producing blue LEDs at Boston University using a novel two-step process.[89] Two years later, in 1993, high-intensity blue LEDs were taken up by Shuji Nakamura of the Nichia Corporation using GaN synthesis processes similar to Moustakas'.[90] Moustakas and Nakamura were assigned separate patents, leading to legal conflicts between them. Nichia and Boston University (especially since, although Moustakas invented his process first, Nakamura registered his before).[91] This new development revolutionized LED lighting, making the manufacture of high-power blue light sources more profitable, leading to the development of technologies such as Blu-ray, and enabling the bright, high-resolution screens of modern tablets and phones.

Nakamura was awarded the Millennium Technology Prize for his contribution to high-power LED technology and high performance.[92] He was also awarded, along with Hiroshi Amano and Isamu Akasaki, the Nobel Prize in Physics in 2014 for his decisive contribution to high-performance LEDs and blue LEDs.[93][94][95][96] In 2015, a US court ruled that three companies (that is, the same plaintiff companies that had not previously resolved their disputes) and that held Nakamura's patents for production in the US, had infringed Moustakas' previous patent and ordered them to pay licensing fees worth $13 million.[97]

By the end of the 90s, blue LEDs were already available. These feature an active region consisting of one or more InGaN quantum wells sandwiched between thicker sheets of GaN, called sheaths. By varying the In/Ga fraction in the InGaN quantum wells, the light emission can, in theory, be shifted from violet to amber. AlGaN gallium aluminum nitride with varying content of the Al/Ga fraction can be used to fabricate the sheath and sheets of quantum wells for UV diodes, but these devices have not yet reached the level of efficiency and technological maturity of blue/green InGaN/GaN devices. If the GaN is used without doping, to form the active layers of the quantum wells the device emits light close to ultraviolet with a peak centered at a wavelength around 365 nm. Green LEDs manufactured in the InGaN/GaN mode are much more efficient and brighter than LEDs produced with non-nitride systems, but these devices still have too low an efficiency for high-brightness applications.

Using aluminum nitrides, such as AlGaN and AlGaInN, even shorter wavelengths are achieved. A range of UV LEDs for different wavelengths are beginning to become available on the market. Near-UV emitting LEDs with wavelengths around 375-395 nm are already sufficiently cheap and can be easily found, for example to replace black light lamps in the inspection of UV anti-counterfeiting watermarks on some documents and on paper money. Shorter wavelength diodes (up to 240 nm),[98]​ are currently on the market, although they are noticeably more expensive.

As the photosensitivity of microorganisms approximately coincides with the absorption spectrum of DNA (with a peak around 260 nm), it is expected to use UV LEDs with emission in the region of 250-270 nm in disinfection and sterilization equipment. Recent research has shown that commercially available UV LEDs (365 nm) are effective in disinfection and sterilization devices.[99]​ UV-C wavelengths were obtained in laboratories using aluminum nitride (210 nm), boron nitride (215 nm), and diamond (235 nm).

RGB

RGB LEDs consist of a red, a blue and a green LED. By independently adjusting each of them, RGB LEDs are capable of producing a wide range of colors. Unlike LEDs dedicated to a single color, RGB LEDs do not produce pure wavelengths. Additionally, commercially available modules are typically not optimized for smooth color blends.

RGB systems

RGB systems.

There are two basic ways to produce white light. One is to use individual LEDs that emit the three primary colors (red, green and blue) and then mix the colors to form white light. The other way is to use a phosphor to convert the monochromatic light from a blue or UV LED into a broad spectrum of white light. It is important to keep in mind that the whiteness of the light produced is essentially designed to satisfy the human eye and depending on each case it may not always be appropriate to think that it is strictly white light. The great variety of whites that are achieved with fluorescent tubes serves as a point of reference.

There are three main methods of producing white light with LEDs.

• - Blue LED + green LED + red LED (mix of colors; although it can be used as a backlight for screens) for lighting they are very poor due to the empty intervals in the frequency spectrum).

• - Near UV LED or UV + RGB phosphor (an LED light that generates a shorter wavelength than blue is used to excite an RGB phosphor).

• - Blue LED + yellow phosphor (two complementary colors combine to produce white light; it is more efficient than the first two methods and is therefore more used in practice).

Due to metamerism "Metamerism (color)"), it is possible to have different spectra that appear white. However, the appearance of objects illuminated by that light can be modified as the spectrum varies. This optical phenomenon is known as color execution, it is different from color temperature, and it makes a truly orange or cyan object appear to be another color and much darker as the LED or the associated phosphor does not emit those wavelengths. The best color reproduction with CFL and LED is achieved by using a mixture of phosphors, which provides lower efficiency, but better light quality. Although the halogen with the highest color temperature is orange, it is still the best artificial light source available in terms of color execution.

White light can be produced by adding lights of different colors; The most common method is the use of red, green and blue (RGB). Hence the method is called multicolor white LEDs (sometimes known as RGB LEDs). Because they require electronic circuitry to control the mixing and diffusion of different colors, and because individual color LEDs have slightly different emission patterns (leading to color variation depending on viewing direction), even if manufactured in a single unit, they are rarely used to produce white light. However, this method has many applications due to the flexibility it presents in producing color mixing[100]​ and, in principle, for offering greater quantum efficiency in the production of white light.

There are several types of multicolor white LEDs: di-, tri- and tetrachromatic white LEDs. Several key factors influence these different realizations, such as color stability, natural color rendering index and luminous efficiency. Frequently, greater luminous efficiency will imply less naturalness of color, thus creating a trade-off between luminous efficiency and naturalness of colors. For example, dichromatic white LEDs have the best luminous efficiency (120 lm/W), but the lowest color rendering capacity. On the other hand, white tetrachromatic LEDs offer excellent color rendering capacity, but are often accompanied by poor luminous efficiency. Trichromatic white LEDs are in an intermediate position, they have good luminous efficiency (> 70 lm/W) and a reasonable capacity for color reproduction.

Phosphor-based LEDs

This method involves coating LEDs of one color (mainly blue InGaN LEDs) with phosphors of different colors to produce white light; The LEDs resulting from the combination are called phosphor-based white LEDs or phosphor converter white LEDs (PCLED). A fraction of blue light undergoes the Stokes shift that transforms shorter wavelengths into longer wavelengths. Depending on the color of the original LED, phosphors of different colors can be used. If several layers of phosphors of different colors are applied, the emission spectrum is broadened, effectively increasing the color rendering index (CRI) value of a given LED.

The efficiency losses of phosphor-based LEDs (with fluorescent substances) are due to heat losses generated by the Stokes shift and also to other degradation problems related to said fluorescent substances. Compared to normal LEDs, their luminous efficiencies depend on the spectral distribution of the resulting light output and the original wavelength of the LED itself. For example, the luminous efficiency of a typical yellow YAG phosphor of a white LED is 3 to 5 times the luminous efficiency of the original blue LED, due to the higher sensitivity of the human eye for the yellow color than for the blue color (depending on the luminosity function model). Due to the simplicity of its manufacture, the phosphor (fluorescent material) method remains the most popular for achieving high intensity white LEDs. The design and production of a light source or lamp using a monochromatic emitter with fluorescent phosphor conversion is simpler and cheaper than a complex RGB system, and most high-intensity white LEDs on the market today are manufactured using fluorescence light conversion.

Among the challenges that arise to improve the efficiency of LED-based white light sources is the development of more efficient fluorescent substances (phosphors). As of 2010, the most efficient yellow phosphorus continues to be YAG phosphorus, which has a Stokes shift loss of less than 10%. Internal optical losses due to reabsorption in the LED chip itself and in the LED encapsulation constitute 10% to 30% of the efficiency loss. Currently, in the field of phosphor development, a great effort is dedicated to its optimization in order to achieve greater light production and higher operating temperatures. For example, efficiency can be increased by better encapsulation design or by using a more suitable type of phosphor. The adjustment coating process is usually used in order to be able to regulate the variable thickness of the phosphor.

Some white phosphor LEDs consist of blue InGaN LEDs encapsulated in an epoxy resin coated with a phosphor. Another option is to associate the LED with a separate phosphor, a prefabricated piece of preformed polycarbonate coated with the phosphor material. Separate phosphors provide more diffuse light, which is favorable for many applications. Designs with separate phosphors are also more tolerant of variations in the LED emission spectrum. A very common yellow phosphorus material is cerium-doped yttrium aluminum garnet (Ce 3+:YAG).

Other white LEDs

Another method used to produce experimental white light LEDs without the use of phosphors is based on the epitaxy of zinc selenide (ZnSe) growth on a ZnSe substrate that simultaneously emits blue light from its active region and yellow light from the substrate.

A new way to produce white LEDs is to use "Wafer (electronic)") wafers composed of gallium nitride on silicon from 200 mm silicon wafers. This avoids the costly fabrication of sapphire substrates from wafers of relatively small sizes, i.e. 100 or 150 mm. The sapphire apparatus must be attached to a mirror-like collector to reflect light, which would otherwise be lost. It is predicted that by 2020, 40% of all GaN LEDs will be made on silicon. Manufacturing large sapphire is difficult, while large silicon material is cheap and more abundant. On the other hand, LED manufacturers who switch from sapphire to silicon must make a minimal investment.

Organic LEDs (OLED)

In an organic light-emitting diode (OLED), the electroluminescent material that makes up the emitting layer of the diode is an organic compound. The organic material is conductive due to electronic delocalization of the pi bonds caused by the conjugated system in all or part of the molecule; Consequently, the material functions as an organic semiconductor. Organic materials can be small organic molecules in the crystalline phase, or polymers.

One of the advantages made possible by OLEDs is thin, low-cost displays with a low supply voltage, a wide viewing angle, high contrast and a wide color gamut. Polymer LEDs have the added advantage of enabling printable and flexible displays. OLEDs have been used in the manufacture of visual displays for portable electronic devices such as mobile phones, digital cameras and MP3 players, and possible future uses are also considered to include lighting and television.

Quantum dot LEDs

At the beginning of the 60s, a decade of technological revolution began with the birth of the Internet and the discovery of LEDs in the visible spectrum. In 1959, the Nobel Prize in Physics Richard P. Feynman, in his famous lecture given at the annual meeting of the Physical Association of the United States titled: "There is a lot of room at the bottom: an invitation to enter a new field of physics", already anticipated the technological revolution and the important discoveries that could involve the manipulation of materials until they were reduced to atomic or molecular sizes or scales.[102] But it was not until the following decade of the 1970s that knowledge of numerous applications became known. of quantum mechanics (about 70 years after its invention) together with the advancement of materials growth and synthesis techniques, represent an important change in the lines of research of numerous groups.[103]​.

Already in this decade, the ability to design structures having new optical and electronic properties was combined with the search for new technological applications to materials already existing in nature. In fact, in 1969, L. Esaki et al. proposed the implementation of heterostructures formed by very thin layers of different materials, giving rise to what is known as engineering and design of energy bands in semiconductor materials.[104]​ The most basic small-dimensional heterostructure is the quantum well (Quantum Well, QW). It consists of a thin layer of a certain semiconductor, of the order of 100 Å, confined between two layers of another semiconductor material characterized by a greater width of the forbidden energy band (bandgap, BG). Due to the small dimensions of the potential well associated with this structure, the carriers are restricted in their movement to a plane perpendicular to the direction of growth. Laser diodes with QWs in the active zone had great advantages, such as the ability to select the emission wavelength based on the width of the well or the decrease in the threshold current, the latter related to the density of states resulting from confinement in a plane.[105]​.

All these advances were followed naturally by others such as the study of systems with confinement in three dimensions, that is, quantum dots (QDs). Thus, QDs can be defined as artificial systems of very small size, from a few tens of nanometers to a few microns in which the carriers are confined in the three directions of three-dimensional space (that is why it is called zero-dimensional), in a region of space smaller than their Broglie wavelength.

When the size of the semiconductor material that constitutes the quantum dot is within the nanometer scale, this material presents a behavior that differs from that observed for it on a macroscopic scale or for the individual atoms that make it up. The electrons in the nanomaterial are restricted to moving in a very small region of space and are said to be confined. When this region is so small that it is comparable to the wavelength associated with the electron (the de Broglie length), then what is called quantum behavior begins to be observed. In these systems, their physical properties are not explained with classical concepts, but are explained through the concepts of quantum mechanics.[106] For example, the minimum potential energy of an electron confined within a nanoparticle is greater than that expected in classical physics and the energy levels of its different electronic states are discrete. Due to quantum confinement, the size of the particle has a fundamental effect on the density of electronic states and therefore, on its optical response. Quantum confinement occurs when the size of the particles has been reduced until it approaches the radius of the Bohr exciton (generating an electron-hole pair or exciton in the semiconductor material), leaving it confined in a very small space. As a consequence, the structure of the energy levels and the optical and electrical properties of the material are considerably modified. The energy levels become discrete and finite, and depend strongly on the size of the nanoparticle.[106]​.

Los ledes se fabrican en una gran variedad de formas y tamaños. El color de la lente de plástico suele coincidir con el de la luz emitida por el led, aunque no siempre es así. Por ejemplo, el plástico de color púrpura se emplea para los ledes infrarrojos y la mayoría de los ledes azules presentan encapsulamientos incoloros. Los ledes modernos de alta potencia como los empleados para iluminación directa o para retroiluminación aparecen normalmente en montajes de tecnología de superficie") (SMT).

Miniature

Miniature LEDs are often used as indicators. In through-hole technology and surface mounts, their size varies from 2 mm to 8 mm. They normally do not have an independent heat sink.[116]​ The maximum current is between 1 mA and 20 mA. Its small size constitutes a limitation in terms of the power consumed due to its high power density and the absence of a heatsink. They are often connected in a daisy chain to form LED light strips.

The most typical plastic cover shapes are round, flat, triangular and square with a flat top. The encapsulation can also be transparent or colored in order to improve contrast and viewing angles.[117]​.

Researchers at the University of Washington have invented the thinnest LED. It is made up of two-dimensional (2-D) materials. Its width is 3 atoms, that is, between 10 and 20 times thinner than three-dimensional (3-D) LEDs and 10,000 times thinner than a human hair. These 2-D LEDs will enable optical communications and smaller, more energy-efficient nano lasers.[118]​.

There are three main categories of single color miniature LEDs:.

Prepared for a current of 2 mA with about 2 V (consumption of more or less 4 mW).

For a current of 20 mA and with 2 or 4-5 V, designed to be seen in direct sunlight. The 5V and 12V LEDs are normal miniature LEDs that incorporate a series resistor for direct connection to a 5 or 12V supply.

High Power

See also: Solid state lighting, LED Lamp, High Power LEDs or HP-LED").

High-power LEDs (HP-LEDs) or high-emission HO-LEDs (High-Output LEDs) can be controlled with currents from hundreds of mA to more than 1 ampere, while other LEDs only reach tens of milliamps. Some can emit more than a thousand lumens.[119]​[120]​.

Power densities of up to 300 W/(cm²) have also been achieved.[121] As overheating of the LEDs can destroy them, they have to be mounted on a heatsink. If the heat from an HP-LED was not transferred to the medium, the device would fail within a few seconds. An HP-LED can replace an incandescent bulb in a flashlight or several of them can be combined to constitute a power LED lamp. Some well-known HP-LEDs in this category They are those of the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon and Cree X-Lamp Since September 2009, there are LEDs manufactured by Cree that exceed 105 lm/W.[122]​.

Examples of Haitz's law, which predicts an exponential increase over time in the light output and efficiency of an LED, are the CREE XP-GE series that reached 105 lm/W in 2009[122]​ and the Nichia 19 series with an average efficiency of 140 lm/W that was launched in 2010.[123]​.

Powered by alternating current

Seoul Semiconductor has developed LEDs that can run on alternating current without the need for a DC converter. In one half cycle, one part of the LED emits light and the other part is dark, and this happens in reverse during the next half cycle. The normal efficiency of this type of HP-LED is 40 lm/W.[124]​ A large number of LED elements in series can operate directly with the mains voltage. In 2009, Seoul Semiconductor launched a high-voltage LED, called 'Acrich MJT', capable of being driven by AC through a simple control circuit. The low power dissipated by these LEDs provides them with greater flexibility than other original AC LED designs.[125]​.

Applications. Variants

The flashing LEDs are used as attention indicators without the need for any type of external electronics. The flashing LEDs look like standard LEDs, but contain an integrated multivibrator circuit that causes the LEDs to flash with a characteristic period of one second. In LEDs fitted with a diffusion lens, this circuit is visible (a small black dot). Most flashing LEDs emit light in a single color, but more sophisticated devices can flash multiple colors and even fade in a sequence of colors from RGB color mixing.

Bi-color LEDs contain two different LEDs in a single assembly. There are two types; The first consists of two dies connected to two conductors parallel to each other with the current circulating in opposite directions. With current flow in one direction, one color is emitted and with current in the opposite direction, the other color is emitted. In the second type, however, the two dies have separate terminals and there is one terminal for each cathode or each anode, so that they can be controlled independently. The most common color combination is traditional red/green, however, there are other combinations available such as traditional green/amber, red/pure green, red/blue or blue/pure green.

Tri-color LEDs contain three different emitting LEDs in a single frame. Each emitter is connected to a separate terminal so that it can be controlled independently of the others. A very characteristic arrangement is in which four terminals appear, a common terminal (the three anodes or the three cathodes joined together) plus an additional terminal for each color.

RGB LEDs are tri-color LEDs with red, green and blue emitters, generally using a four-wire connection and a common terminal (anode or cathode). This type of LEDs can have both the positive terminal and the negative terminal as common. Other models, however, only have two terminals (positive and negative) and a small built-in electronic control unit.

This type of LED has emitters of different colors and is equipped with only two output terminals. The colors are switched internally by varying the supply voltage.

Alphanumeric LEDs are available as seven-segment displays, fourteen-segment displays, or dot matrix displays. Seven-segment displays can represent all numbers and a limited set of letters while fourteen-segment displays can display all letters. Dot matrix displays typically use 5x7 pixels per character. The use of seven-segment LEDs became widespread in the 1970s and 1980s, but the increasing use of liquid crystal displays has reduced the popularity of numeric and alphanumeric LEDs due to their lower power requirements and greater display flexibility.

They are RGB LEDs that contain their own "smart" control electronics. In addition to power and ground, they have connections for data input and output, and sometimes for clock or strobe signals. They are connected in a daisy chain, with the data input to the first LED equipped with a microprocessor that can control the brightness and color of each of them, independently of the others. They are used where a combination is necessary that provides maximum control and a minimum view of the electronics, such as in Christmas light chains or LED matrices. Some even feature refresh rates in the kHz range, making them suitable for basic video applications.

Power supplies

Main article: Circuit with LED.

The current-voltage characteristic curve of an LED is similar to that of other diodes, in which the current intensity (or briefly, current) grows exponentially with the voltage "Voltage (electricity)") (see Shockley's equation). This means that a small change in voltage can cause a large change in current.[129]​ If the applied voltage exceeds the forward bias threshold voltage drop of the LED, by a small amount, the current limit that the diode can withstand can be greatly exceeded, potentially damaging or destroying the LED. The solution that can be adopted to avoid this is to use constant current intensity power sources (briefly, constant current source[130]​) capable of keeping the current below the maximum value of the current that the LED can pass through or, at least, if a conventional constant voltage source "Voltage (electricity)") or battery is used, add a limiting resistor in series with the LED to the LED lighting circuit. Since normal power sources (batteries, mains) are normally constant voltage sources, most LED fixtures must include a power converter or at least a current limiting resistor. However, the high resistance of three-volt button cells combined with the high differential resistance of nitride-derived LEDs makes it possible to power such LEDs with a button cell without the need to incorporate an external resistor.

electrical polarity

Main article: Electrical polarity of LEDs.

As with all diodes, current flows easily from the p-type material to the n-type material.[131] However, if a small voltage is applied in the reverse direction, current does not flow and no light is emitted. If the reverse voltage rises enough to exceed the breakdown voltage, a high current flows and the LED may be damaged. If the reverse current is limited enough to prevent damage, the reverse driving LED can be used as an avalanche diode.

Health and safety

The vast majority of devices containing LEDs are "safe under normal use", and are therefore classified as "Risk Product 1 RG1 (low risk)" / "LED Class 1". Currently, only a few LEDs—extremely bright LEDs that have a very small viewing angle of an aperture of 8° or less—could, in theory, cause temporary blindness and are therefore classified as "Risk 2 RG2 (moderate risk)."[132]​ LED technology is also used in the healthcare sector to reduce energy consumption costs and slow the spread of hospital-acquired infections. Its use is also being studied for the treatment of pain, insomnia and other disorders and diseases, among others, Alzheimer's.[133]​.

The opinion of the French Agency for Food, Environmental and Occupational Health and Safety (ANSES), when addressing health issues related to LEDs in 2010, suggested prohibiting the public use of lamps that were in Group 2 or Moderate Risk, especially those with a high blue component, in places frequented by children.[134]​.

In general, safety regulations for the use of laser light[135]​[136]​—and Risk 1, Risk 2 devices, etc.—are also applicable to LEDs.[137]​.

Just as LEDs have the advantage, over fluorescent lamps, that they do not contain mercury "Mercury (element)"), however, they may contain other dangerous metals such as lead and arsenic. Regarding the toxicity of LEDs when treated as waste, a study published in 2011 stated: "According to federal regulations, LEDs are not hazardous, except for low-intensity red LEDs, because they initially contained Pb (lead) in concentrations above the regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), lead (up to 8103 mg/kg, limit: 1000), nickel (up to 4797 mg/kg, limit: 2000), or silver (up to 721 mg/kg, limit: 500) cause all LEDs, except low intensity yellow ones, to be dangerous."[138]

Indicators and signal lamps

The low energy consumption, low need for maintenance and small size of LEDs has led to their use as status and display indicators in a wide variety of equipment and installations. Large-area LED screens are used to broadcast the game in stadiums, as dynamic decorative screens and as dynamic message signs on highways. Light, thin message displays are used in airports and railway stations and as destination information panels on trains, buses, trams and ferries.

Single-color lights are suitable for traffic lights, traffic signs, "Emergency Exit (Safety)" exit signs, vehicle emergency lighting, navigation lights, headlights (the standard chromaticity and luminance indices were established in the International Convention for the Prevention of Collisions at Sea 1972 Annex 1 and by the International Commission on Illumination or CIE) and Christmas lights composed of LEDs. In regions with cold climates, LED traffic lights may remain covered in snow.[164] Red or yellow LEDs are used in indicators and alphanumeric displays, in environments where night vision must be maintained: airplane cockpits, underwater and ship bridges, astronomical observatories and in the field, for example for the observation of animals at night and military field applications.

Given their long lifespan, fast switching times, and ability to be seen in broad daylight due to their high intensity and concentration, LEDs have been used for some time for brake lights in cars, trucks, and buses, and in direction change signals; Many vehicles currently use LEDs in their rear light assemblies. The use in brakes improves safety due to the great reduction in the time required for a complete ignition, that is, due to the fact that it has a shorter rise time, up to 0.5 seconds faster than an incandescent bulb. This provides more reaction time for the drivers behind. In a two-intensity circuit (rear marker lights and brake lights) if the LEDs are not actuated at a fast enough frequency, they can create a ghost array, where LED ghost images will appear if the eyes move quickly across the light array. Headlights with white LEDs are beginning to be used. The use of LEDs has stylistic advantages because they can form much thinner beams of light than incandescent lamps fitted with parabolic reflectors.

Low-power LEDs are relatively very economical and allow their use in short-lived luminous objects such as luminous self-adhesives, disposable objects and the Lumalive photonic fabric. Artists also use LEDs for so-called LED art. Weather and distress radio receivers with coded area messages (SAME) have three LEDs: red for alarms, orange for attention and yellow for warnings, indications and reports.

Lightning

To encourage the switch to LED lamps, the United States Department of Energy has created the L Prize. The Philips Lighting North America LED bulb won the first prize on August 3, 2011 after successfully completing 18 months of intensive field, laboratory and product testing.[165]​.

LEDs are used as street lights and in architectural lighting. Mechanical robustness and long lifespan are used in automotive lighting in cars, motorcycles and bicycle lights. LED light emission can be effectively controlled by using non-imaging optical principles.

In 2007, the Italian town of Torraca was the first place to convert its entire lighting system to LEDs.[166] LEDs are also used in aviation, Airbus has used LED lighting on its Airbus A320 since 2007, and Boeing uses LED lighting on the 787. LEDs are also now used in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline lights, and edge lights.

LEDs are also used as a light source for DLP projectors and to illuminate LCD televisions (known as LED televisions) and laptop screens. RGB LEDs increase the color range by up to 45%. TV screens and computer screens can be made thinner by using LEDs for backlighting.[167]​ The lack of infrared or thermal radiation makes LEDs ideal for stage lighting with banks of RGB LEDs that can easily change color and decrease lighting warm-up, as well as medical lighting where IR radiation can be harmful. In energy conservation, there is less heat production when using LEDs.

They are also small, durable and require little power, which is why they are used in portable devices such as flashlights. LED strobes or "Flash (photography)" camera flashes operate at a safe, low voltage, rather than the 250+ volts commonly found in xenon flash based lighting. This is especially useful in cell phone cameras. LEDs are used for infrared illumination in night vision applications including security cameras. A ring of LEDs around a forward-facing video camera on a retroreflective background enables chroma keying in video productions.

LEDs are used in mining operations, as cap lamps to provide light to miners. Research has been conducted to improve mining LEDs, reduce glare and increase illumination, reducing the risk of injury to miners.[168]​.

LEDs are now commonly used in all market areas, from commercial to domestic use: standard lighting, theatrical, architectural, public installations, and wherever artificial light is used.

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement, and new technologies such as AmBX, exploiting the versatility of LEDs. NASA has even sponsored research into the use of LEDs to promote health for astronauts.[169]​.

Optical communications. Data transfer and other communications

Light can be used to transmit data and analog signals. For example, white LEDs can be used in systems to help people orient themselves in closed spaces in order to locate arrangements or objects.[170]​.

"Assisted listening devices" in many theaters and similar spaces use arrays of infrared LEDs to send sound to spectators' receivers. LEDs (and also semiconductor lasers) are used to send data over many types of fiber optic cable. From TOSLINK cables for digital audio transmission to the very high-bandwidth fiber links that form the backbone of the Internet. For a time, computers were equipped with IrDA interfaces, allowing them to send and receive data of nearby equipment using infrared radiation.

Because LEDs can turn on and off millions of times per second, they require very high bandwidth for data transmission.[171]​[172]​.

Sustainable lighting

Lighting efficiency is necessary for sustainable architecture. In 2009, tests carried out with LED bulbs by the United States Department of Energy showed an average efficiency from 35 lm/W, therefore below the efficiency of CFLs, to values ​​as low as 9 lm/W, worse than incandescent bulbs. A typical 13-watt LED bulb emitted 450 to 650 lumens,[173]​ which was equivalent to a standard 40-watt incandescent bulb.

In any case, in 2011 there were LED bulbs with an efficiency of 150 lm/W, and even low-end models exceeded 50 lm/W, so a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The latter have a durability of 1,000 hours while an LED can continue operating at a lower efficiency for more than 50,000 hours.[174]​.

Comparative table of led-LFC-incandescent bulb:.

The reduction in electrical energy consumption achieved with LED-based lighting is important when compared to incandescent lighting. Furthermore, this reduction also manifests itself as a notable decrease in damage to the environment. Each country presents a different energy panorama and, therefore, although the impact on energy consumption is the same, the production of gases harmful to the environment may fluctuate somewhat from one country to another. Regarding consumption, a conventional 40-watt incandescent bulb can be taken as an example. An equivalent light output can be obtained with a 6 watt LED system. By using the LED system instead of incandescent bulbs, energy consumption can be reduced by more than 85%. Regarding the savings in environmental impact, it is possible to quantify it for any country if the CO production for each kW per hour is known. In the specific case of Spain, it is known that the energy mix of the Spanish electricity grid has produced about 308 g of CO/kWh in 2016. It is assumed for the calculation that both the bulb and the LED assembly have operated for 10 hours a day throughout the year 2016.[175]​ The energies consumed have been 146 kW-hour by the incandescent bulb and 21.6 kW-hour by part of the led set. The electrical energy consumed can be translated into kg of CO produced per year. In the first case, the generation of about 45 kg of CO has been carried out, while in the second case the production of CO has been reduced to 6.75 kg.

Light sources for artificial vision systems

Industrial vision systems usually require homogeneous lighting to be able to focus on image features of interest. This is one of the most frequent uses of LED lights, and it will surely continue to do so, driving down the prices of systems based on light signaling. Barcode scanners are the most common example of vision systems; many of these low-cost products use LEDs instead of lasers.[176] Optical computer mice also use LEDs for their vision system, as they provide a uniform light source over the surface for the miniature camera inside the mouse. In fact, LEDs are an almost ideal light source for vision systems for the following reasons:.

• - The size of the illuminated field is usually comparatively small and machine vision systems are often quite expensive, so the cost of the light source is usually less of a concern. However, it may not be easy to replace a broken light source within complex machinery; In this case the long lifespan of the LEDs is a benefit.

• - LED components tend to be small and can be placed at high density on flat or uniform surface substrates (PCB, etc.) so that homogeneous light sources can be designed that direct light from strictly controlled directions onto inspected parts. This can often be achieved with small, low-cost lenses and diffusers, helping to achieve high light densities with control over illumination levels and homogeneity. LED sources can be configured in various ways (spotlights for reflective lighting, ring lights for coaxial lighting, backlights for contour lighting, linear mounts, large format flat panels, dome sources for diffuse omnidirectional lighting).

• - LEDs can be easily stroboscopic (in the microsecond range and below) and image synchronized. High power LEDs are available to allow well-illuminated images, even with very short light pulses. This is often used to get sharp, sharp images of fast-moving parts.

• - LEDs come in various colors and wavelengths, allowing easy use of the best color for each need, where the different color can provide better visibility of features of interest. Having a precisely known spectrum allows closely matched filters to be used to separate the informational bandwidth or to reduce the disturbing effects of ambient light. LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation. This allows the use of plastic lenses, filters and diffusers. Waterproof units can also be easily designed, allowing use in harsh or humid environments (food, beverage, oil industries).

Medicine and biology

Healthcare has echoed the advantages of LEDs over other types of lighting and has incorporated them into its latest generation equipment. The advantages offered by LEDs in their current state of development have led to their rapid diffusion in the world of instruments for diagnosis and support in medical and surgical procedures. The advantages appreciated by medical professionals are the following:.

• - The small size of the light sources that, in general, can be associated with very thin and flexible light guides, which allows them to move inside thin catheters.

• - The lack of accompanying infrared radiation, which allows the adjective cold light to be associated with them. The heat given off by other types of light sources made their use difficult or impossible in certain diagnostic observations or surgical interventions.

• - The white tone that is usually the favorite for medical observations. It must be a natural white color capable of presenting all colors without metamerism problems. The natural color of the tissues illuminated in this way favors the correct diagnosis of the observed field.

• - The high luminous intensity achievable by these light sources.

Based on the previous ideas, current endoscopes are equipped with LED lighting. The endoscopic technique covers many medical specialties, for example gastroscopy, colonoscopy, laryngoscopy, otoscopy or arthroscopy. All of these techniques allow the observation of organs and systems of the human body through the use of miniature video cameras. They can also be used in surgical interventions or to make diagnoses. The equipment is also known as videoscopes or videoendoscopes. There are rigid or flexible ones depending on the needs. Fiber optics adapt to each particular case. On the other hand, the lighting in operating rooms and dental clinics are currently LED. They perfectly satisfy all the technical and health requirements for their use. Particularly appreciated is the obtaining of bright, natural, white lighting (more than one hundred and fifty thousand candelas one meter away from the field of operation), without shadows and without infrared or ultraviolet emissions that could affect both the patient and the medical staff participating in the intervention.

The same thing happens with the headlamps of surgeons and dentists equipped with LEDs, with lamps for medical examinations, for ophthalmological examinations and interventions or for minor surgery, so it can be said that LEDs have come to cover all medical specialties. Optical companies dedicated to medicine have incorporated LEDs into their observation equipment, for example in microscopes, thereby obtaining many advantages for the study of images using different techniques (bright field, contrast, fluorescence), which is evident in the advertising and commercial fields. LEDs are successfully used as sensors in heart rate monitors or oxygen blood pressure monitors to measure oxygen saturation.

Industry

The industry has adapted the observation models used in medicine for its own needs and the equipment is called "industrial endoscopes" or also borescopes), flexoscopes) or video endoscopes). With them you can observe the interior of machines, engines, ducts, cavities or weapons without having to dismantle them.

Other applications

The light from LEDs can be modulated very quickly, which is why they are widely used in fiber optics and free space optical communication. This includes remote controls used in LED televisions, VCRs and computers. Optical isolators use an LED combined with a photodiode or phototransistor to provide an electrically isolated signal path between two circuits. This is especially useful in medical equipment where the signals from a low voltage sensor circuit (typically battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits that do not share a common ground potential.

Many sensor systems rely on light as a signal source. LEDs are often ideal as a light source due to sensor requirements. LEDs are used as motion sensors"), for example in optical computer mice "Mouse (computing)"). The Nintendo Wii Sensor Bar uses infrared LEDs. Pulse oximeters use them to measure oxygen saturation. Some tabletop scanners use RGB LED arrays instead of the typical cold cathode fluorescent lamp as a light source. Having independent control of three illuminated colors allows the scanner to be calibrated for a more accurate color balance and not There is no need for heating. In addition, their sensors only need to be monochromatic, since at any time the scanned page is only illuminated with one color of light. Since LEDs can also be used as photodiodes, they can also be used for photo emission or detection. This could be used, for example, in a touch screen that records light reflected from a finger or a stylus. photosynthesis in plants,[178]​ and bacteria and viruses can be eliminated from water and other substances using UV LEDs for sterilization.

LEDs have also been used as a quality voltage reference in electronic circuits. Instead of a Zener diode in low voltage regulators, forward voltage drop can be used (for example, about 1.7 V for a normal red LED). Red LEDs have the flattest I/V curve. Although the LED forward voltage is much more current dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available.

The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable for incorporation into thin materials, has encouraged experimentation in combining light sources and interior wall covering surfaces.[179]​ The new possibilities offered by these developments have led some designers and companies, such as Meystyle"),[180]​ Ingo Maurer,[181]​ Lomox,[182]​ and Philips[183]​ to research and develop proprietary LED wallpaper technologies, some of which are currently available for commercial purchase. Other solutions exist primarily as prototypes or are in the process of being refined.

• - OLED.

• - Plasma screen.

• - Wikimedia Commons hosts a multimedia category on led.

• - Wikimedia Commons hosts a multimedia category on led (SMD) "commons:Category:Light-emitting diodes (SMD)").

• - Wiktionary has definitions and other information about LED.

• - on YouTube.