Semiconductor and solid-state light source

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This article is about the electronic device. For specific use in lighting, see LED lamp

"LED" and "Led" redirect here. For other uses, see LED (disambiguation)

Parts of a conventional LED. The flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out via mechanical strain or vibration.

Close-up of an LED with the voltage being increased and decreased to show a detailed view of its operation

A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor.[5] White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.[6]

Appearing as practical electronic components in , the earliest LEDs emitted low-intensity infrared (IR) light.[7] Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red.

Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Later developments produced LEDs available in visible, ultraviolet (UV), and infrared wavelengths with high, low, or intermediate light output, for instance, white LEDs suitable for room and outdoor lighting. LEDs have also given rise to new types of displays and sensors, while their high switching rates are useful in advanced communications technology with applications as diverse as aviation lighting, fairy lights, strip lights, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper, horticultural grow lights, and medical devices.[8]

LEDs have many advantages over incandescent light sources, including lower power consumption, a longer lifetime, improved physical robustness, smaller sizes, and faster switching. In exchange for these generally favorable attributes, disadvantages of LEDs include electrical limitations to low voltage and generally to DC (not AC) power, the inability to provide steady illumination from a pulsing DC or an AC electrical supply source, and a lesser maximum operating temperature and storage temperature.

LEDs are transducers of electricity into light. They operate in reverse of photodiodes, which convert light into electricity.

History

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Discoveries and early devices

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Green electroluminescence from a point contact on a crystal of SiC recreates Round's original experiment from . Close-up of a 1 watt red power led

Electroluminescence as a phenomenon was discovered in by the English experimenter Henry Joseph Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector.[9][10] Russian inventor Oleg Losev reported the creation of the first LED in .[11] His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades, partly due to the very inefficient light-producing properties of silicon carbide, the semiconductor Losev used.[12][13]

SMD (Surface Mount Device) package LED

In , Georges Destriau observed that electroluminescence could be produced when zinc sulphide (ZnS) powder is suspended in an insulator and an alternating electrical field is applied to it. In his publications, Destriau often referred to luminescence as Losev-Light. Destriau worked in the laboratories of Madame Marie Curie, also an early pioneer in the field of luminescence with research on radium.[14][15]

Hungarian Zoltán Bay together with György Szigeti patenting a lighting device in Hungary in based on silicon carbide, with an option on boron carbide, that emitted white, yellowish white, or greenish white depending on impurities present.[16] Kurt Lehovec, Carl Accardo, and Edward Jamgochian explained these first LEDs in using an apparatus employing SiC crystals with a current source of a battery or a pulse generator and with a comparison to a variant, pure, crystal in .[17][18]

Rubin Braunstein[19] of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in .[20] Braunstein observed infrared emission generated by simple diode structures using gallium antimonide (GaSb), GaAs, indium phosphide (InP), and silicon-germanium (SiGe) alloys at room temperature and at 77 kelvins. In , Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. As noted by Kroemer[21] Braunstein "&#;had set up a simple optical communications link: Music emerging from a record player was used via suitable electronics to modulate the forward current of a GaAs diode. The emitted light was detected by a PbS diode some distance away. This signal was fed into an audio amplifier and played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of fun playing with this setup."

A Texas Instruments SNX-100 GaAs LED contained in a TO-18 transistor metal case

In September , while working at Texas Instruments in Dallas, Texas, James R. Biard and Gary Pittman discovered near-infrared (900 nm) light emission from a tunnel diode they had constructed on a GaAs substrate.[7] By October , they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically isolated semiconductor photodetector.[22] On August 8, , Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc-diffused p&#;n junction LED with a spaced cathode contact to allow for efficient emission of infrared light under forward bias.

After establishing the priority of their work based on engineering notebooks predating submissions from G.E. Labs, RCA Research Labs, IBM Research Labs, Bell Labs, and Lincoln Lab at MIT, the U.S. patent office issued the two inventors the patent for the GaAs infrared light-emitting diode (U.S. Patent US), the first practical LED.[7] Immediately after filing the patent, Texas Instruments (TI) began a project to manufacture infrared diodes. In October , TI announced the first commercial LED product (the SNX-100), which employed a pure GaAs crystal to emit an 890 nm light output.[7] In October , TI announced the first commercial hemispherical LED, the SNX-110.[23]

In the s, several laboratories focused on LEDs that would emit visible light. A particularly important device was demonstrated by Nick Holonyak on October 9, , while he was working for General Electric in Syracuse, New York. The device used the semiconducting alloy gallium phosphide arsenide (GaAsP). It was the first semiconductor laser to emit visible light, albeit at low temperatures. At room temperature it still functioned as a red light-emitting diode. GaAsP was the basis for the first wave of commercial LEDs emitting visible light. It was mass produced by the Monsanto and Hewlett-Packard companies and used widely for displays in calculators and wrist watches.[24][25][26]

SMD (Surface Mount Device) package red LED

M. George Craford,[27] a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in .[28] In , T. P. Pearsall designed the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.[29]

Close-up of Red LED

Initial commercial development

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LED display of a TI-30 scientific calculator (c.

&#;

), which uses plastic lenses to increase the visible digit size X-Ray of a s 8-digit LED calculator display

Until , visible and infrared LEDs were extremely costly, on the order of US$200 per unit, and so had little practical use.[30] The first commercial visible-wavelength LEDs used GaAsP semiconductors and were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as calculators, TVs, radios, telephones, as well as watches.[31]

The Hewlett-Packard company (HP) was engaged in research and development (R&D) on practical LEDs between and , by a research team under Howard C. Borden, Gerald P. Pighini at HP Associates and HP Labs.[32] During this time HP collaborated with Monsanto Company on developing the first usable LED products.[33] The first usable LED products were HP's LED display and Monsanto's LED indicator lamp, both launched in .[33]

Monsanto was the first organization to mass-produce visible LEDs, using Gallium arsenide phosphide (GaAsP) in to produce red LEDs suitable for indicators.[30] Monsanto had previously offered to supply HP with GaAsP, but HP decided to grow its own GaAsP.[30] In February , Hewlett-Packard introduced the HP Model - Numeric Indicator, the first LED device to use integrated circuit (integrated LED circuit) technology.[32] It was the first intelligent LED display, and was a revolution in digital display technology, replacing the Nixie tube and becoming the basis for later LED displays.[34]

In the s, commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the planar process (developed by Jean Hoerni,[35][36] ). The combination of planar processing for chip fabrication and innovative packaging methods enabled the team at Fairchild led by optoelectronics pioneer Thomas Brandt to achieve the needed cost reductions.[37] LED producers have continued to use these methods as of about .[38]

The early red LEDs were bright enough for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors became widely available and appeared in appliances and equipment.

Early LEDs were packaged in metal cases similar to those of transistors, with a glass window or lens to let the light out. Modern indicator LEDs are packed in transparent molded plastic cases, tubular or rectangular in shape, and often tinted to match the device color. Infrared devices may be dyed, to block visible light. More complex packages have been adapted for efficient heat dissipation in high-power LEDs. Surface-mounted LEDs further reduce the package size. LEDs intended for use with fiber optics cables may be provided with an optical connector.

Blue LED

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The first blue-violet LED, using magnesium-doped gallium nitride was made at Stanford University in by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering.[39][40] At the time Maruska was on leave from RCA Laboratories, where he collaborated with Jacques Pankove on related work. In , the year after Maruska left for Stanford, his RCA colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc-doped gallium nitride, though the subsequent device Pankove and Miller built, the first actual gallium nitride light-emitting diode, emitted green light.[41][42]

In the U.S. Patent Office awarded Maruska, Rhines, and Stanford professor David Stevenson a patent for their work in (U.S. Patent US A). Today, magnesium-doping of gallium nitride remains the basis for all commercial blue LEDs and laser diodes. In the early s, these devices were too dim for practical use, and research into gallium nitride devices slowed.

In August , Cree introduced the first commercially available blue LED, based on the indirect bandgap semiconductor, silicon carbide (SiC).[43] SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.[44][45]

In the late s, key breakthroughs in GaN epitaxial growth and p-type doping[46] ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, Theodore Moustakas at Boston University patented a method for producing high-brightness blue LEDs using a new two-step process in .[47] In , a US court ruled that three Taiwanese companies had infringed Moustakas's prior patent, and ordered them to pay licensing fees of not less than US$13 million.[48]

Two years later, in , high-brightness blue LEDs were demonstrated by Shuji Nakamura of Nichia Corporation using a gallium nitride (GaN) growth process.[49][50][51] These LEDs had efficiencies of 10%.[52] In parallel, Isamu Akasaki and Hiroshi Amano of Nagoya University were working on developing the important GaN deposition on sapphire substrates and the demonstration of p-type doping of GaN. This new development revolutionized LED lighting, making high-power blue light sources practical, leading to the development of technologies like Blu-ray.[53][54]

Nakamura was awarded the Millennium Technology Prize for his invention.[55] Nakamura, Hiroshi Amano, and Isamu Akasaki were awarded the Nobel Prize in Physics in for "the invention of efficient blue light-emitting diodes, which has enabled bright and energy-saving white light sources."[56]

In , Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED using indium tin oxide (ITO) on (AlGaInP/GaAs).

In [57] and ,[58] processes for growing gallium nitride (GaN) LEDs on silicon were successfully demonstrated. In January , Osram demonstrated high-power InGaN LEDs grown on silicon substrates commercially,[59] and GaN-on-silicon LEDs are in production at Plessey Semiconductors. As of , some manufacturers are using SiC as the substrate for LED production, but sapphire is more common, as it has the most similar properties to that of gallium nitride, reducing the need for patterning the sapphire wafer (patterned wafers are known as epi wafers). Samsung, the University of Cambridge, and Toshiba are performing research into GaN on Si LEDs.

Toshiba has stopped research, possibly due to low yields.[60][61][62][63][64][65][66] Some opt for epitaxy, which is difficult on silicon, while others, like the University of Cambridge, choose a multi-layer structure, in order to reduce (crystal) lattice mismatch and different thermal expansion ratios, to avoid cracking of the LED chip at high temperatures (e.g. during manufacturing), reduce heat generation and increase luminous efficiency. Sapphire substrate patterning can be carried out with nanoimprint lithography.[67][68][69][70][71][72][73]

GaN-on-Si is difficult but desirable since it takes advantage of existing semiconductor manufacturing infrastructure. It allows for the wafer-level packaging of LED dies resulting in extremely small LED packages.[74]

GaN is often deposited using metalorganic vapour-phase epitaxy (MOCVD),[75] and it also uses lift-off.

White LEDs and the illumination breakthrough

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Even though white light can be created using individual red, green and blue LEDs, this results in poor color rendering, since only three narrow bands of wavelengths of light are being emitted. The attainment of high efficiency blue LEDs was quickly followed by the development of the first white LED. In this device a Y
3Al
5O
12:Ce (known as "YAG" or Ce:YAG phosphor) cerium-doped phosphor coating produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. Using different phosphors produces green and red light through fluorescence. The resulting mixture of red, green and blue is perceived as white light, with improved color rendering compared to wavelengths from the blue LED/YAG phosphor combination.[76]

Illustration of Haitz's law, showing improvement in light output per LED over time, with a logarithmic scale on the vertical axis

The first white LEDs were expensive and inefficient. The light output then increased exponentially. The latest research and development has been propagated by Japanese manufacturers such as Panasonic and Nichia, and by Korean and Chinese manufacturers such as Samsung, Solstice, Kingsun, Hoyol and others. This trend in increased output has been called Haitz's law after Roland Haitz.[77][78]

Light output and efficiency of blue and near-ultraviolet LEDs rose and the cost of reliable devices fell. This led to relatively high-power white-light LEDs for illumination, which are replacing incandescent and fluorescent lighting.[79][80]

Experimental white LEDs were demonstrated in to produce 303 lumens per watt of electricity (lm/W); some can last up to 100,000 hours.[81][82] Commercially available LEDs have an efficiency of up to 223 lm/W as of .[83][84][85] A previous record of 135 lm/W was achieved by Nichia in .[86] Compared to incandescent bulbs, this is a huge increase in electrical efficiency, and even though LEDs are more expensive to purchase, overall lifetime cost is significantly cheaper than that of incandescent bulbs.[87]

The LED chip is encapsulated inside a small, plastic, white mold[88][89] although sometimes an LED package can incorporate a reflector.[90] It can be encapsulated using resin (polyurethane-based), silicone,[91][92][93] or epoxy[94] containing (powdered) Cerium-doped YAG phosphor particles.[95] The viscosity of phosphor-silicon mixtures must be carefully controlled.[95] After application of a phosphor-silicon mixture on the LED using techniques such as jet dispensing,[96] and allowing the solvents to evaporate, the LEDs are often tested, and placed on tapes for SMT placement equipment for use in LED light bulb production. Some "remote phosphor" LED light bulbs use a single plastic cover with YAG phosphor for one[97] or several blue LEDs, instead of using phosphor coatings on single-chip white LEDs.[98] Ce:YAG phosphors and epoxy in LEDs[99] can degrade with use, and is more apparent with higher concentrations of Ce:YAG in phosphor-silicone mixtures, because the Ce:YAG decomposes with use.[100][101][102]

The output of LEDs can shift to yellow over time due to degradation of the silicone.[92] There are several variants of Ce:YAG, and manufacturers in many cases do not reveal the exact composition of their Ce:YAG offerings.[103] Several other phosphors are available for phosphor-converted LEDs to produce several colors such as red, which uses nitrosilicate phosphors,[104][105] and many other kinds of phosphor materials exist for LEDs such as phosphors based on oxides, oxynitrides, oxyhalides, halides, nitrides, sulfides, quantum dots, and inorganic-organic hybrid semiconductors. A single LED can have several phosphors at the same time.[96][106] Some LEDs use phosphors made of glass-ceramic or composite phosphor/glass materials.[107][108] Alternatively, the LED chips themselves can be coated with a thin coating of phosphor-containing material, called a conformal coating.[109][110]

The temperature of the phosphor during operation and how it is applied limits the size of an LED die. Wafer-level packaged white LEDs allow for extremely small LEDs.[74]

Polychromatic

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In , QPixel introduced as polychromatic LED that could replace the 3-subpixel model for digital displays. The technology uses a gallium nitride semiconductor that emits light of different frequencies modulated by voltage changes. A prototype display achieved a resolution of 6,800 PPI or 3k x 1.5k pixels.[111]

Physics of light production and emission

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In a light-emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "electroluminescence". The wavelength of the light depends on the energy band gap of the semiconductors used. Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light.[112]

Unlike a laser, the light emitted from an LED is neither spectrally coherent nor even highly monochromatic. Its spectrum is sufficiently narrow that it appears to the human eye as a pure (saturated) color.[113][114] Also unlike most lasers, its radiation is not spatially coherent, so it cannot approach the very high intensity characteristic of lasers.

Single-color LEDs

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By selection of different semiconductor materials, single-color LEDs can be made that emit light in a narrow band of wavelengths from near-infrared through the visible spectrum and into the ultraviolet range. The required operating voltages of LEDs increase as the emitted wavelengths become shorter (higher energy, red to blue), because of their increasing semiconductor band gap.

Blue LEDs have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber.

Aluminium gallium nitride (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If unalloyed GaN is used in this case to form the active quantum well layers, the device emits near-ultraviolet light with a peak wavelength centred around 365 nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.[citation needed]

With AlGaN and AlGaInN, even shorter wavelengths are achievable. Near-UV emitters at wavelengths around 360&#;395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in documents and bank notes, and for UV curing. Substantially more expensive, shorter-wavelength diodes are commercially available for wavelengths down to 240 nm.[115] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED emitting at 250&#;270 nm are expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices.[116] UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm),[117] boron nitride (215 nm)[118][119] and diamond (235 nm).[120]

White LEDs

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There are two primary ways of producing white light-emitting diodes. One is to use individual LEDs that emit three primary colors&#;red, green and blue&#;and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, similar to a fluorescent lamp. The yellow phosphor is cerium-doped YAG crystals suspended in the package or coated on the LED. This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through in LEDs with partial phosphor conversion. Alternatively, white LEDs may use other phosphors like manganese(IV)-doped potassium fluorosilicate (PFS) or other engineered phosphors. PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor.

In LEDs with PFS phosphor, some blue light passes through the phosphors, the Ce:YAG phosphor converts blue light to green and red (yellow) light, and the PFS phosphor converts blue light to red light. The color, emission spectrum or color temperature of white phosphor converted and other phosphor converted LEDs can be controlled by changing the concentration of several phosphors that form a phosphor blend used in an LED package.[121][122][123][124]

The 'whiteness' of the light produced is engineered to suit the human eye. Because of metamerism, it is possible to have quite different spectra that appear white. The appearance of objects illuminated by that light may vary as the spectrum varies. This is the issue of color rendition, quite separate from color temperature. An orange or cyan object could appear with the wrong color and much darker as the LED or phosphor does not emit the wavelength it reflects. The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering.[citation needed]

The first white light-emitting diodes (LEDs) were offered for sale in the autumn of .[125] Nichia made some of the first white LEDs which were based on blue LEDs with Ce:YAG phosphor.[126] Ce:YAG is often grown using the Czochralski method.[127]

RGB systems

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Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. FWHM spectral bandwidth is approximately 24&#;27 nm for all three colors. An RGB LED projecting red, green, and blue onto a surface

Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors. Since LEDs have slightly different emission patterns, the color balance may change depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors,[128] and in principle, this mechanism also has higher quantum efficiency in producing white light.[129]

There are several types of multicolor white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.[130]

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of few green LEDs exceed even 100 lumens per watt. The blue and red LEDs approach their theoretical limits.[citation needed]

Multicolor LEDs offer a means to form light of different colors. Most perceivable colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. Their emission power decays exponentially with rising temperature,[131] resulting in a substantial change in color stability. Such problems inhibit industrial use. Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. LEDs without phosphor, while a poorer solution for general lighting, are the best solution for displays, either backlight of LCD, or direct LED based pixels.

Dimming a multicolor LED source to match the characteristics of incandescent lamps is difficult because manufacturing variations, age, and temperature change the actual color value output. To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color.[132]

Phosphor-based LEDs

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Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband Stokes-shifted light emitted by the Ce3+:YAG phosphor, which emits at roughly 500&#;700 nm

This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs).[133] A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Depending on the original LED's color, various color phosphors are used. Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the color rendering index (CRI).[134]

Phosphor-based LEDs have efficiency losses due to heat loss from the Stokes shift and also other phosphor-related issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function).

Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.[citation needed]

1 watt 9 volt three chips SMD phosphor based white LED

Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of , the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.[citation needed]

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce3+:YAG).[citation needed]

White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.[citation needed]

A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes.[135] The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since , 40% of all GaN LEDs are made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.[136]

Mixed white LEDs

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Tunable white LED array in a floodlight

There are RGBW LEDs that combine RGB units with a phosphor white LED on the market. Doing so retains the extremely tunable color of RGB LED, but allows color rendering and efficiency to be optimized when a color close to white is selected.[137]

Some phosphor white LED units are "tunable white", blending two extremes of color temperatures (commonly K and K) to produce intermediate values. This feature allows users to change the lighting to suit the current use of a multifunction room.[138] As illustrated by a straight line on the chromaticity diagram, simple two-white blends will have a pink bias, becoming most severe in the middle. A small amount of green light, provided by another LED, could correct the problem.[139] Some products are RGBWW, i.e. RGBW with tunable white.[140]

A final class of white LED with mixed light is dim-to-warm. These are ordinary K white LED bulbs with a small red LED that turns on when the bulb is dimmed. Doing so makes the color warmer, emulating an incandescent light bulb.[140]

Other white LEDs

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Another method used to produce experimental white light LEDs used no phosphors at all and was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.[141]

Organic light-emitting diodes (OLEDs)

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In an organic light-emitting diode (OLED), the electroluminescent material composing the emissive layer of the diode is an organic compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of the molecule, and the material therefore functions as an organic semiconductor.[142] The organic materials can be small organic molecules in a crystalline phase, or polymers.[143]

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut.[144] Polymer LEDs have the added benefit of printable and flexible displays.[145][146][147] OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, lighting and televisions.[143][144]

Types

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LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern high-power LEDs such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages (not shown).

LEDs are made in different packages for different applications. A single or a few LED junctions may be packed in one miniature device for use as an indicator or pilot lamp. An LED array may include controlling circuits within the same package, which may range from a simple resistor, blinking or color changing control, or an addressable controller for RGB devices. Higher-powered white-emitting devices will be mounted on heat sinks and will be used for illumination. Alphanumeric displays in dot matrix or bar formats are widely available. Special packages permit connection of LEDs to optical fibers for high-speed data communication links.

Miniature

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Image of miniature surface mount LEDs in most common sizes. They can be much smaller than a traditional 5

 

mm lamp type LED, shown on the upper left corner. Very small (1.6×1.6×0.35

 

mm) red, green, and blue surface mount miniature LED package with gold wire bonding details

These are mostly single-die LEDs used as indicators, and they come in various sizes from 1.8 mm to 10 mm, through-hole and surface mount packages.[148] Typical current ratings range from around 1 mA to above 20 mA. LED's can be soldered to a flexible PCB strip to form LED tape popularly used for decoration.

Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle. Infrared devices may have a black tint to block visible light while passing infrared radiation, such as the Osram SFH .[149]

5 V and 12 V LEDs are ordinary miniature LEDs that have a series resistor for direct connection to a 5 V or 12 V supply.[150]

High-power light-emitting diodes attached to an LED star base (Luxeon, Lumileds)

High-power LEDs (HP-LEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.[151][152] LED power densities up to 300 W/cm2 have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP-LED is not removed, the device fails in seconds. One HP-LED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.

Some HP-LEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September , some HP-LEDs manufactured by Cree exceed 105 lm/W.[153]

Examples for Haitz's law&#;which predicts an exponential rise in light output and efficacy of LEDs over time&#;are the CREE XP-G series LED, which achieved 105 lm/W in [153] and the Nichia 19 series with a typical efficacy of 140 lm/W, released in .[154]

LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficiency of this type of HP-LED is typically 40 lm/W.[155] A large number of LED elements in series may be able to operate directly from line voltage. In , Seoul Semiconductor released a high DC voltage LED, named 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.[156]

Strip

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This section is an excerpt from LED strip light

Several LED spots being reflected as continuous lighting strip

An LED strip, tape, or ribbon light is a flexible circuit board populated by surface-mount light-emitting diodes (SMD LEDs) and other components that usually comes with an adhesive backing. Traditionally, strip lights had been used solely in accent lighting, backlighting, task lighting, and decorative lighting applications, such as cove lighting.

LED strip lights originated in the early s. Since then, increased

LED strip lights originated in the early s. Since then, increased luminous efficacy and higher-power SMDs have allowed them to be used in applications such as high brightness task lighting, fluorescent and halogen lighting fixture replacements, indirect lighting applications, ultraviolet inspection during manufacturing processes, set and costume design, and growing plants.

Composite image of an

11 × 44

LED matrix lapel name tag display using /-type SMD LEDs. Top: A little over half of the

21 × 86 mm

display. Center: Close-up of LEDs in ambient light. Bottom: LEDs in their own red light.

Considerations for use

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• Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.

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  The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
• Size: LEDs can be very small (smaller than 2 mm2

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  ) and are easily attached to printed circuit boards.

Power sources

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Simple LED circuit with resistor for current limiting

The current in an LED or other diodes rises exponentially with the applied voltage (see Shockley diode equation), so a small change in voltage can cause a large change in current. Current through the LED must be regulated by an external circuit such as a constant current source to prevent damage. Since most common power supplies are (nearly) constant-voltage sources, LED fixtures must include a power converter, or at least a current-limiting resistor. In some applications, the internal resistance of small batteries is sufficient to keep current within the LED rating.[citation needed]

For more information, please visit Joineonlux.

LEDs are sensitive to voltage. They must be supplied with a voltage above their threshold voltage and a current below their rating. Current and lifetime change greatly with a small change in applied voltage. They thus require a current-regulated supply (usually just a series resistor for indicator LEDs).[166]

Efficiency droop: The efficiency of LEDs decreases as the electric current increases. Heating also increases with higher currents, which compromises LED lifetime. These effects put practical limits on the current through an LED in high power applications.[167]

Electrical polarity

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Unlike a traditional incandescent lamp, an LED will light only when voltage is applied in the forward direction of the diode. No current flows and no light is emitted if voltage is applied in the reverse direction. If the reverse voltage exceeds the breakdown voltage, which is typically about five volts, a large current flows and the LED will be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.[citation needed]

By definition, the energy band gap of any diode is higher when reverse-biased than when forward-biased. Because the band gap energy determines the wavelength of the light emitted, the color cannot be the same when reverse-biased. The reverse breakdown voltage is sufficiently high that the emitted wavelength cannot be similar enough to still be visible. Though dual-LED packages exist that contain a different color LED in each direction, it is not expected that any single LED element can emit visible light when reverse-biased.[citation needed]

It is not known if any zener diode could exist that emits light only in reverse-bias mode. Uniquely, this type of LED would conduct when connected backwards.

Appearance

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• Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
• Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
• Color rendition: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can make the color of objects appear differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,

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  red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs. The same is true with green surfaces. The quality of color rendition of an LED is measured by the Color Rendering Index (CRI).
• Dimming: LEDs can be dimmed either by pulse-width modulation or lowering the forward current.

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  This pulse-width modulation is why LED lights, particularly headlights on cars, when viewed on camera or by some people, seem to flash or flicker. This is a type of stroboscopic effect.

Light properties

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• Switch on time: LEDs light up extremely quickly. A typical red indicator LED achieves full brightness in under a microsecond.

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  LEDs used in communications devices can have even faster response times.
• Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages total internal reflection (TIR) lenses are often used to the same effect. When large quantities of light are needed, many light sources such as LED chips are usually deployed, which are difficult to focus or collimate on the same target.
• Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So, LEDs are difficult to apply to uses needing a spherical light field. Different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees.

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Reliability

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• Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.

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• Thermal runaway: Parallel strings of LEDs will not share current evenly due to the manufacturing tolerances in their forward voltage. Running two or more strings from a single current source may result in LED failure as the devices warm up. If forward voltage binning is not possible, a circuit is required to ensure even distribution of current between parallel strands.

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• Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.

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• Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be shorter or longer.

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  Fluorescent tubes typically are rated at about 10,000 to 25,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.

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• Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike incandescent and fluorescent lamps that fail faster when cycled often, or high-intensity discharge lamps (HID lamps) that require a long time to warm up to full output and to cool down before they can be lighted again if they are being restarted.
• Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment &#; or thermal management properties. Overdriving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and require low failure rates.

Manufacturing

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LED manufacturing involves multiple steps, including epitaxy, chip processing, chip separation, and packaging.[177]

In a typical LED manufacturing process, encapsulation is performed after probing, dicing, die transfer from wafer to package, and wire bonding or flip chip mounting,[178] perhaps using indium tin oxide, a transparent electrical conductor. In this case, the bond wire(s) are attached to the ITO film that has been deposited in the LEDs.

Flip chip circuit on board (COB) is a technique that can be used to manufacture LEDs.[179]

Colors and materials

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Conventional LEDs are made from a variety of inorganic semiconductor materials, the following table shows the available colors with wavelength range, voltage drop and material:

Applications

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Daytime running light LEDs of an automobile

LED uses fall into five major categories:

The application of LEDs in horticulture has revolutionized plant cultivation by providing energy-efficient, customizable lighting solutions that optimize plant growth and development.[192] LEDs offer precise control over light spectra, intensity, and photoperiods, enabling growers to tailor lighting conditions to the specific needs of different plant species and growth stages. This technology enhances photosynthesis, improves crop yields, and reduces energy costs compared to traditional lighting systems. Additionally, LEDs generate less heat, allowing closer placement to plants without risking thermal damage, and contribute to sustainable farming practices by lowering carbon footprints and extending growing seasons in controlled environments.[193] Light spectrum affects growth, metabolite profile, and resistance against fungal phytopathogens of Solanum lycopersicum seedlings.[194] LEDs can also be used in micropropagation.[195]

Indicators and signs

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The low energy consumption, low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED displays are used as stadium displays, dynamic decorative displays, and dynamic message signs on freeways. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.

Red and green LED traffic signals

One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights, and LED-based Christmas lights

Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs have been used in automotive brake lights and turn signals. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, about 0.1 second faster[citation needed] than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED appear if the eyes quickly scan across the array. White LED headlamps are beginning to appear. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with parabolic reflectors.

Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as glowsticks and throwies. Artists have also used LEDs for LED art.

Lighting

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With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. To encourage the shift to LED lamps and other high-efficiency lighting, in the US Department of Energy created the L Prize competition. The Philips Lighting North America LED bulb won the first competition on August 3, , after successfully completing 18 months of intensive field, lab, and product testing.[196]

Efficient lighting is needed for sustainable architecture. As of , some LED bulbs provide up to 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W, so that a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The lower heat output of LEDs also reduces demand on air conditioning systems. Worldwide, LEDs are rapidly adopted to displace less effective sources such as incandescent lamps and CFLs and reduce electrical energy consumption and its associated emissions. Solar powered LEDs are used as street lights and in architectural lighting.

The mechanical robustness and long lifetime are used in automotive lighting on cars, motorcycles, and bicycle lights. LED street lights are employed on poles and in parking garages. In , the Italian village of Torraca was the first place to convert its street lighting to LEDs.[197]

Cabin lighting on recent[when?] Airbus and Boeing jetliners uses LED lighting. LEDs are also being used in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.

LEDs are also used as a light source for DLP projectors, and to backlight newer LCD television (referred to as LED TV), computer monitor (including laptop) and handheld device LCDs, succeeding older CCFL-backlit LCDs although being superseded by OLED screens. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.[198]

LEDs are small, durable and need little power, so they are used in handheld devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flashlamp-based lighting. This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions.

LED for miners, to increase visibility inside mines Los Angeles Vincent Thomas Bridge illuminated with blue LEDs

LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners.[199]

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement.[200] NASA has even sponsored research for the use of LEDs to promote health for astronauts.[201]

Data communication and other signalling

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Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.[202]

Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of fiber optic cable, from digital audio over TOSLINK cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.

Because LEDs can cycle on and off millions of times per second, very high data bandwidth can be achieved.[203] For that reason, visible light communication (VLC) has been proposed as an alternative to the increasingly competitive radio bandwidth.[204] VLC operates in the visible part of the electromagnetic spectrum, so data can be transmitted without occupying the frequencies of radio communications.

Machine vision systems

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Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used.

Barcode scanners are the most common example of machine vision applications, and many of those scanners use red LEDs instead of lasers. Optical computer mice use LEDs as a light source for the miniature camera within the mouse.

LEDs are useful for machine vision because they provide a compact, reliable source of light. LED lamps can be turned on and off to suit the needs of the vision system, and the shape of the beam produced can be tailored to match the system's requirements.

Biological detection

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The discovery of radiative recombination in aluminum gallium nitride (AlGaN) alloys by U.S. Army Research Laboratory (ARL) led to the conceptualization of UV light-emitting diodes (LEDs) to be incorporated in light-induced fluorescence sensors used for biological agent detection.[205][206][207] In , the Edgewood Chemical Biological Center (ECBC) initiated the effort to create a biological detector named TAC-BIO. The program capitalized on semiconductor UV optical sources (SUVOS) developed by the Defense Advanced Research Projects Agency (DARPA).[207]

UV-induced fluorescence is one of the most robust techniques used for rapid real-time detection of biological aerosols.[207] The first UV sensors were lasers lacking in-field-use practicality. In order to address this, DARPA incorporated SUVOS technology to create a low-cost, small, lightweight, low-power device. The TAC-BIO detector's response time was one minute from when it sensed a biological agent. It was also demonstrated that the detector could be operated unattended indoors and outdoors for weeks at a time.[207]

Aerosolized biological particles fluoresce and scatter light under a UV light beam. Observed fluorescence is dependent on the applied wavelength and the biochemical fluorophores within the biological agent. UV induced fluorescence offers a rapid, accurate, efficient and logistically practical way for biological agent detection. This is because the use of UV fluorescence is reagentless, or a process that does not require an added chemical to produce a reaction, with no consumables, or produces no chemical byproducts.[207]

Additionally, TAC-BIO can reliably discriminate between threat and non-threat aerosols. It was claimed to be sensitive enough to detect low concentrations, but not so sensitive that it would cause false positives. The particle-counting algorithm used in the device converted raw data into information by counting the photon pulses per unit of time from the fluorescence and scattering detectors, and comparing the value to a set threshold.[208]

The original TAC-BIO was introduced in , while the second-generation TAC-BIO GEN II, was designed in to be more cost-efficient, as plastic parts were used. Its small, light-weight design allows it to be mounted to vehicles, robots, and unmanned aerial vehicles. The second-generation device could also be utilized as an environmental detector to monitor air quality in hospitals, airplanes, or even in households to detect fungus and mold.[209][210]

Other applications

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LED costume for stage performers LED wallpaper by Meystyle A large LED display behind a disc jockey Seven-segment display that can display four digits and points LED panel light source used in an early experiment on potato growth during Shuttle mission STS-73 to investigate the potential for growing food on future long duration missions

The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. This includes remote controls, such as for television sets, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage sensor circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also lets information be transferred between circuits that do not share a common ground potential.

Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. The Nintendo Wii's sensor bar uses infrared LEDs. Pulse oximeters use them for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light.

Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a touchscreen that registers reflected light from a finger or stylus.[211] Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase photosynthesis in plants,[212] and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization.[116] LEDs of certain wavelengths have also been used for light therapy treatment of neonatal jaundice and acne.[213]

UV LEDs, with spectra range of 220 nm to 395 nm, have other applications, such as water/air purification, surface disinfection, glue curing, free-space non-line-of-sight communication, high performance liquid chromatography, UV curing dye printing, phototherapy (295nm Vitamin D, 308nm Excimer lamp or laser replacement), medical/ analytical instrumentation, and DNA absorption.[206][214]

LEDs have also been used as a medium-quality voltage reference in electronic circuits. The forward voltage drop (about 1.7 V for a red LED or 1.2V for an infrared) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far 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 to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls in the form of LED wallpaper.

Research and development

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Key challenges

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LEDs require optimized efficiency to hinge on ongoing improvements such as phosphor materials and quantum dots.[215]

The process of down-conversion (the method by which materials convert more-energetic photons to different, less energetic colors) also needs improvement. For example, the red phosphors that are used today are thermally sensitive and need to be improved in that aspect so that they do not color shift and experience efficiency drop-off with temperature. Red phosphors could also benefit from a narrower spectral width to emit more lumens and becoming more efficient at converting photons.[216]

In addition, work remains to be done in the realms of current efficiency droop, color shift, system reliability, light distribution, dimming, thermal management, and power supply performance.[215]

Early suspicions were that the LED droop was caused by elevated temperatures. Scientists showed that temperature was not the root cause of efficiency droop.[217] The mechanism causing efficiency droop was identified in as Auger recombination, which was taken with mixed reaction.[167] A study conclusively identified Auger recombination as the cause.[218]

Potential technology

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A new family of LEDs are based on the semiconductors called perovskites. In , less than four years after their discovery, the ability of perovskite LEDs (PLEDs) to produce light from electrons already rivaled those of the best performing OLEDs.[219] They have a potential for cost-effectiveness as they can be processed from solution, a low-cost and low-tech method, which might allow perovskite-based devices that have large areas to be made with extremely low cost. Their efficiency is superior by eliminating non-radiative losses, in other words, elimination of recombination pathways that do not produce photons; or by solving outcoupling problem (prevalent for thin-film LEDs) or balancing charge carrier injection to increase the EQE (external quantum efficiency). The most up-to-date PLED devices have broken the performance barrier by shooting the EQE above 20%.[220]

In , Cao et al. and Lin et al. independently published two papers on developing perovskite LEDs with EQE greater than 20%, which made these two papers a mile-stone in PLED development. Their device have similar planar structure, i.e. the active layer (perovskite) is sandwiched between two electrodes. To achieve a high EQE, they not only reduced non-radiative recombination, but also utilized their own, subtly different methods to improve the EQE.[220]

In the work of Cao et al.,[221] researchers targeted the outcoupling problem, which is that the optical physics of thin-film LEDs causes the majority of light generated by the semiconductor to be trapped in the device.[222] To achieve this goal, they demonstrated that solution-processed perovskites can spontaneously form submicrometre-scale crystal platelets, which can efficiently extract light from the device. These perovskites are formed via the introduction of amino acid additives into the perovskite precursor solutions. In addition, their method is able to passivate perovskite surface defects and reduce nonradiative recombination. Therefore, by improving the light outcoupling and reducing nonradiative losses, Cao and his colleagues successfully achieved PLED with EQE up to 20.7%.[221]

Lin and his colleague used a different approach to generate high EQE. Instead of modifying the microstructure of perovskite layer, they chose to adopt a new strategy for managing the compositional distribution in the device&#;an approach that simultaneously provides high luminescence and balanced charge injection. In other words, they still used flat emissive layer, but tried to optimize the balance of electrons and holes injected into the perovskite, so as to make the most efficient use of the charge carriers. Moreover, in the perovskite layer, the crystals are perfectly enclosed by MABr additive (where MA is CH3NH3). The MABr shell passivates the nonradiative defects that would otherwise be present perovskite crystals, resulting in reduction of the nonradiative recombination. Therefore, by balancing charge injection and decreasing nonradiative losses, Lin and his colleagues developed PLED with EQE up to 20.3%.[223]

Health and safety

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Certain blue LEDs and cool-white LEDs can exceed safe limits of the so-called blue-light hazard as defined in eye safety specifications such as "ANSI/IESNA RP-27.1&#;05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems".[224] One study showed no evidence of a risk in normal use at domestic illuminance,[225] and that caution is only needed for particular occupational situations or for specific populations.[226] In , the International Electrotechnical Commission published IEC Photobiological safety of lamps and lamp systems, replacing the application of early laser-oriented standards for classification of LED sources.[227]

While LEDs have the advantage over fluorescent lamps, in that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic.[228]

In the American Medical Association (AMA) issued a statement concerning the possible adverse influence of blueish street lighting on the sleep-wake cycle of city-dwellers. Industry critics claim exposure levels are not high enough to have a noticeable effect.[229]

Environmental issues

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See also

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References

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Further reading

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LED Lighting Supply

The History of LED Lights

The History of LED Lights

Last Updated Nov 24,
Published on Jul 19,

Written by Neil Peterson Last Updated Nov 24, Published on Jul 19,

These days, LED lighting can be found just about everywhere. The light-emitting diode (LED) is currently the most energy-efficient lighting technology available. It is one of the most rapidly developing technologies around and it is used in a wide variety of applications. You can find installed in large commercial warehouses. Powerful stadium lights are used for baseball, softball, and football fields. LEDs are used in homes, businesses, automobiles, agricultural equipment, and more.

But did you know that its history of development stretches back over a hundred years? Before we go back to LED lighting&#;s past, let&#;s understand how it currently functions.

How Does it Work?

Instead of a filament, each LED lamp or fixture contains multiple light-emitting diodes. The light that it generates projects in one direction as opposed to 360 degrees from the source. This eliminates the need to use reflectors that are typically used with other sources of light such as fluorescent and halogen.

The story of the invention of the LED dates to some of the earliest days of wireless technology. Very little was understood about semiconductors at the time. Even less was understood about using them to emit light. LEDs have been commercially available since the s, but the birth of the LED stretches many years before this. The invention of LEDs has its roots in much earlier developments that weren&#;t even related to lighting technology. Let&#;s take a journey back in time to explore the history of LED lights and the history of LED light bulbs.

The Complete Timeline of LED Lighting History

The s

In , electroluminescence was discovered by who was a British radio engineer.

Early radio detectors were typically created utilizing crystals of what would later be called semiconductors. A thin wire would be placed onto the detector&#;s surface and then a point-contact diode would be made. These were called "Cat's Whiskers". Round observed that when an electrical current was passed through some of the detectors, one of them emitted light. However, the light was too faint to be of much use for anything.

The s

In ,

Losev continued his studies of what would later become known as semiconductor technology until . In , he would be killed in the Siege of Leningrad during World War Two. Because of this, the records of his work were mostly destroyed and they would not reemerge until the s.

The s

In , a French physicist named would first use the word &#;electroluminescence&#;. This came from a paper that he published about the occurrence of light being generated by Zinc Sulphide powder from the conduction of an electric current through it.

The s

In , the development of the transistor leads to a big step forward in semiconductor physics. This made it possible to better explain the process of light emission. Because of this, new diodes were invented and developed.

In , the first green LED was created at RCA Laboratories by

The s

In ,

In , the red LED was developed by

In , IBM began to use LEDs for the first time on circuit boards in early computers. They were used as indicators on equipment. This was also around the time that LEDs began to be manufactured. One of the first companies to do this was Monsanto which was a supplier of raw semiconductor materials.

In , Hewlett Packard began to incorporate LEDs into its calculators.

The s

In and , blue, orange, yellow, violet, and green LEDs were developed using new semiconductor materials. Also at this time, LEDs are invented that are much brighter than earlier versions. Monsanto begins to manufacture LED lights on a large scale.

In , high brightness LEDs that can be used for fiber optic applications were developed by Thomas P. Pearsall. This would improve communications technology on a global scale.

The s

In ,

The s

From -,

The s

In , white LEDs for residential applications became available on the commercial market for the first time.
In , LED light efficiency reached 100 lumens per watt. This would be more efficient at the time than any other type of light (apart from high-intensity discharge (HID) lights).

The s

By , there were LED lights being produced that could reach an efficiency of 250 lumens per watt. Nothing else could match it. By , LED lights were becoming the main source of lighting for practically all applications. Other types of lighting such as incandescent, fluorescent, and halogen were slowly being phased out.

Conclusion

The development of LED lighting has progressed over a 100+ year period. It began slowly, in fits and starts, but it quickly developed over the past 30 years. Even more so over the past 10 years.

Today, LED lights have a distinct advantage over other types of lighting in every way.

• They are more efficient, which saves on energy costs.


• They last longer. Up to 100,000 hours, in some cases.


• Almost no energy is wasted on heat.


• They are environmentally friendly because they contain no toxic elements such as mercury. This makes for easy disposal.


• They deliver a higher quality of light that does not degrade over their lifespan.

The advancement of LED lighting technology is still ongoing. Here are some recent developments:

• Glare-reduction headlights for cars also optimize illumination for the driver.


• Li-Fi networks that provide an alternate internet connection outside of standard Wi-Fi.


• Human-centric LED lighting that mimics the color spectrum of the sun.

It&#;s impossible to know how far the advancement of LEDs will continue to go. What we do know is that LED lights will eventually phase out all other types of lighting. If (and until) a better lighting technology comes along, it will be our primary source of artificial light for years to come.

 

About the Author

Neil Peterson is Chief Operating Officer at LED Lighting Supply. He has been active in the LED industry for over 10 years and is responsible for product planning and management as well as revenue and operations at LED Lighting Supply. Much of Neil&#;s time is focused on customer engagement for large commercial and industrial lighting requirements. When not working, he enjoys family time, camping, fishing, and sports.

These days, LED lighting can be found just about everywhere. The light-emitting diode (LED) is currently the most energy-efficient lighting technology available. It is one of the most rapidly developing technologies around and it is used in a wide variety of applications. You can findinstalled in large commercial warehouses. Powerful stadium lights are used for baseball, softball, and football fields. LEDs are used in homes, businesses, automobiles, agricultural equipment, and more.But did you know that its history of development stretches back over a hundred years? Before we go back to LED lighting&#;s past, let&#;s understand how it currently functions. LED lighting is a type of solid-state lighting (SSL) in which a semiconductor transforms electricity to light in a small space (the diode) of about one square millimeter or less. LEDs create light by electroluminescence in a semiconductor material. Electroluminescence is the phenomenon of a material emitting light when an electric current or an electric field is passed through it.Instead of a filament, each LED lamp or fixture contains multiple light-emitting diodes. The light that it generates projects in one direction as opposed to 360 degrees from the source. This eliminates the need to use reflectors that are typically used with other sources of light such as fluorescent and halogen.The story of the invention of the LED dates to some of the earliest days of wireless technology. Very little was understood about semiconductors at the time. Even less was understood about using them to emit light. LEDs have been commercially available since the s, but the birth of the LED stretches many years before this. The invention of LEDs has its roots in much earlier developments that weren&#;t even related to lighting technology. Let&#;s take a journey back in time to explore the history of LED lights and the history of LED light bulbs.In , electroluminescence was discovered bywho was a British radio engineer.Early radio detectors were typically created utilizing crystals of what would later be called semiconductors. A thin wire would be placed onto the detector&#;s surface and then a point-contact diode would be made. These were called "Cat's Whiskers". Round observed that when an electrical current was passed through some of the detectors, one of them emitted light. However, the light was too faint to be of much use for anything.In , Oleg Losev , who was a Russian physicist, observed Round&#;s discovery regarding electrical currents and inorganic material and called it the &#;Round Effect&#;. In , Losev published a report that contained theories on his findings from continued studies of electroluminescence and light-emitting diodes in radio sets.Losev continued his studies of what would later become known as semiconductor technology until . In , he would be killed in the Siege of Leningrad during World War Two. Because of this, the records of his work were mostly destroyed and they would not reemerge until the s.In , a French physicist namedwould first use the word &#;electroluminescence&#;. This came from a paper that he published about the occurrence of light being generated by Zinc Sulphide powder from the conduction of an electric current through it.In , the development of the transistor leads to a big step forward in semiconductor physics. This made it possible to better explain the process of light emission. Because of this, new diodes were invented and developed.In , the first green LED was created at RCA Laboratories by Rubin Braunstein and Egon Loebner.In , Gary Pittman and James R. Biard discovered and developed the Infrared LED while working for Texas Instruments.In , the red LED was developed by Nick Holonyak at General Electric Laboratories. It is the first of its kind on the visible wavelength.In , IBM began to use LEDs for the first time on circuit boards in early computers. They were used as indicators on equipment. This was also around the time that LEDs began to be manufactured. One of the first companies to do this was Monsanto which was a supplier of raw semiconductor materials.In , Hewlett Packard began to incorporate LEDs into its calculators.In and , blue, orange, yellow, violet, and green LEDs were developed using new semiconductor materials. Also at this time, LEDs are invented that are much brighter than earlier versions. Monsanto begins to manufacture LED lights on a large scale.In , high brightness LEDs that can be used for fiber optic applications were developed by Thomas P. Pearsall. This would improve communications technology on a global scale.In , Walden C. Rhines and Herbert Maruska created a blue LED using Magnesium at Stafford University. The development would establish future standards for LED lighting. It would set the stage for high-brightness LEDs that could be used in other applications besides indicator lights.From -, Shuji Nakamura along with two other colleagues (Isamu Akasaki and Hiroshi Amano), using Gallium Nitride, invented ultra-bright blue LEDs. Just afterward, using Indium Gallium Nitride, he developed high-intensity green and blue LEDs. This was a very significant development for the future of LED lighting. The ultra-bright blue LEDs laid the groundwork for later development of cost-efficient and very functional white LED lights. Fixtures and bulbs that use white LED lights are now what you see installed in residential, commercial, and manufacturing sectors in the present day. White LED lights first appeared on the market in .In , white LEDs for residential applications became available on the commercial market for the first time.In , LED light efficiency reached 100 lumens per watt. This would be more efficient at the time than any other type of light (apart from high-intensity discharge (HID) lights).By , there were LED lights being produced that could reach an efficiency of 250 lumens per watt. Nothing else could match it. By , LED lights were becoming the main source of lighting for practically all applications. Other types of lighting such as incandescent, fluorescent, and halogen were slowly being phased out.The development of LED lighting has progressed over a 100+ year period. It began slowly, in fits and starts, but it quickly developed over the past 30 years. Even more so over the past 10 years.Today, LED lights have a distinct advantage over other types of lighting in every way.The advancement of LED lighting technology is still ongoing. Here are some recent developments:It&#;s impossible to know how far the advancement of LEDs will continue to go. What we do know is that LED lights will eventually phase out all other types of lighting. If (and until) a better lighting technology comes along, it will be our primary source of artificial light for years to come.

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