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A light-emitting diode (LED) is a semiconductor diode that emits coherence (physics) narrow-spectrum light when electrically Voltage bias in the forward direction of the p-n junction. This effect is a form of electroluminescence.

An LED is usually a small area source, often with extra optics added to the chip that shapes its radiation pattern. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible spectrum, or near-ultraviolet. An LED can be used as a regular household light source.

History In the early 20th century, Henry Round of Marconi Labs first noted that a semiconductor junction would produce light. Russian Oleg Vladimirovich Losev independently created the first LED in the mid 1920s; his research, though distributed in Russian, German and British scientific journals, was ignored. Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide and other semiconductor alloys in 1955. Experimenters at Texas Instruments, Bob Biard and Gary Pittman, found in 1961 that gallium arsenide gave off infrared (invisible) light when electric current was applied. Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting diode. Nick Holonyak, then of the General Electric Company and later with the University of Illinois at Urbana-Champaign, developed the first practical visible-spectrum LED in 1962 and is seen as the "father of the light-emitting diode". Holonyak's former graduate student, M. George Craford, invented in 1972 the first yellow LED and 10x brighter red and red-orange LEDs.

Shuji Nakamura of Nichia of Japan demonstrated the first high-brightness blue LED based on Indium gallium nitride, borrowing on critical developments in Gallium nitride nucleation on sapphire substrates and the demonstration of p-type doping of GaN which were developed by I. Akasaki and H. Amano in Nagoya. The existence of the blue LED led quickly to the first white LED, which employed a Y3Al5O12:Ce, or "YAG", phosphor coating to mix yellow (down-converted) light with blue to produce light that appears white. Nakamura was awarded the 2006 Millennium Technology Prize for his invention.

LED technology Physical function Like a normal diode, an LED consists of a chip of semiconducting material impregnated, or Doping (semiconductor), with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and electron hole—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for an LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The refractive index of the package material should match the index of the semiconductor, otherwise the produced light gets partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat, thus lowering the efficiency. This type of reflection also occurs at the surface of the package if the LED is coupled to a medium with a different refractive index such as a glass fiber or air. The refractive index of most LED semiconductors is quite high, so in almost all cases the LED is coupled into a much lower-index medium. The large index difference makes the reflection quite substantial (per the Fresnel equations), and this is usually one of the dominant causes of LED inefficiency. Often more than half of the emitted light is reflected back at the LED-package and package-air interfaces. The reflection is most commonly reduced by using a dome-shaped (half-sphere) package with the diode in the center so that the outgoing light rays strike the surface perpendicularly, at which angle the reflection is minimized. An anti-reflection coating may be added as well. The package may be cheap plastic, which may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted. Other strategies for reducing the impact of the interface reflections include designing the LED to reabsorb and reemit the reflected light (called photon recycling) and manipulating the microscopic structure of the surface to reduce the reflectance, either by introducing random roughness or by creating programmed moth eye surface patterns.

Conventional LEDs are made from a variety of inorganic semiconductor materials, producing the following colors:

Aluminium gallium arsenide (AlGaAs) — red and infrared
Aluminium gallium phosphide (AlGaP) — green
Aluminium gallium indium phosphide (AlGaInP) — high-brightness orange-red, orange, yellow, and green
Gallium arsenide phosphide (GaAsP) — red, orange-red, Orange (color), and yellow
Gallium phosphide (GaP) — red, yellow and green
Gallium nitride (GaN) — green, pure green (or emerald green), and blue also white (if it has an AlGaN Quantum Barrier)
Indium gallium nitride (InGaN) — near ultraviolet, bluish-green and blue
Silicon carbide (SiC) as substrate — blue
Silicon (Si) as substrate — blue (under development)
Sapphire (Al2O3) as substrate — blue
Zinc selenide (ZnSe) — blue
Diamond (C) — ultraviolet
Aluminium nitride (AlN), aluminium gallium nitride (AlGaN), aluminium gallium indium nitride (AlGaInN) — near to far ultraviolet (down to 210 nm)

With this wide variety of colors, arrays of multicolor LEDs can be designed to produce unconventional color patterns.

Ultraviolet and blue LEDs GaN LED.

Blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle.

The first blue LEDs were made in 1971 by Jacques Pankove (inventor of the gallium nitride LED) at RCA.{{cite web| title = Alumni society honors four leaders in engineering and technology | work = Berkeley Engineering News | publisher = | date = [2000-09-04 | url = http://www.coe.berkeley.edu/EPA/EngNews/00F/EN2F/deaa.html | format = | doi = | accessdate = 2007-01-23 --> However, these devices were too feeble to be of much practical use. In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping by Akasaki and Amano (Nagoya, Japan) ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in high brightness blue LEDs were demonstrated through the work of [Shuji Nakamura at [Nichia Corporation.{{cite web | title = United States Patent No. 5,578,839 (Nakamura et al.) | work = | publisher = [United States Patent and Trademark Office | date = filed [1993-11-17 | url = http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F5578839 | format = | doi = | accessdate = 2007-01-23 -->

By the late 1990s, blue LEDs had become widely available. They 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 InN-GaN fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN aluminum gallium nitride of varying AlN 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 the InGaN-GaN blue/green devices. If the active quantum well layers are GaN, as opposed to alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350–370 nm. Green LEDs manufactured from the InGaN-GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.

With aluminium containing nitrides, most often Aluminium gallium nitride and aluminium gallium indium nitride, even shorter wavelengths are achievable. Ultraviolet LEDs are becoming available on the market, in a range of wavelengths. Near-UV emitters at wavelengths around 375–395 nm are already cheap, common to encounter e.g., as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and paper currencies. Shorter wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 247 nm. Sensor Electronic Technology, Inc.: Nitride Products Manufacturer As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with peak at about 260 nm, UV LEDs emitting at 250–270 nm are prospective for disinfecting devices.

Wavelengths down to 210 nm were obtained in laboratories using aluminium nitride.

While not actually an LED as such, an ordinary NPN bipolar transistor will emit violet light if its emitter-base junction is subjected to non-destructive reverse breakdown. This is easy to demonstrate by filing the top off a metal-can transistor (BC107, 2N2222 or similar) and biasing it well above emitter-base breakdown (≥ 20 V) via a current limiting resistor.

White LEDs Blue LEDs can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle. Most "white" LEDs in production today are based on an InGaN-GaN structure, and emit blue light of wavelengths between 450 nm and 470 nm blue GaN. These GaN-based, InGaN-active-layer LEDs are covered by a yellowish phosphor coating usually made of cerium-Doping (Semiconductors) YAG (Ce3+:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce3+:YAG. The single crystal form of Ce3+:YAG is actually considered a scintillator rather than a phosphor. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the color vision of white, the resulting shade often called "lunar white". This approach was developed by Nichia and was used by them from 1996 for manufacturing of white LEDs.

The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminum in the YAG with gallium. Due to the spectral characteristics of the diode, the red and green colors of objects in its blue yellow light are not as vivid as in broad-spectrum light. Manufacturing variations and varying thicknesses in the phosphor make the LEDs produce light with different color temperatures, from warm yellowish to cold bluish; the LEDs have to be sorted during manufacture by their actual characteristics. Philips Lumileds Lighting Company patented conformal coating process addresses the issue of varying phosphor thickness, giving the white LEDs a more consistent spectrum of white light.

ed light emitted by the Ce3+:YAG phosphor which extends from around 500 to 700 nanometers.

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium-based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu, Al). This is a method analogous to the way fluorescent lamps work. However the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore 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 approaches offer comparable brightness.

The newest method used to produce white light LEDs uses no phosphors at all and is based on wiktionary:homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate.

A new technique developed by Michael Bowers, a graduate student at Vanderbilt University in Nashville, involves coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This technique produces a warm, yellowish-white light similar to that produced by incandescent bulbs.{{cite news| title = Accidental Invention Points to End of Light Bulbs | publisher =LiveScience.com | date = October 21 [ | url = http://www.livescience.com/technology/051021_nano_light.html | accessdate = 2007-01-24 -->

Organic light-emitting diodes (OLEDs) spectral bandwidth is approximately 24–27 nanometres for all three colors.

If the emitting layer material of an LED is an organic compound, it is known as an Organic Light Emitting Diode (Organic light-emitting diode). To function as a semiconductor, the organic emitting material must have conjugated system. The emitting material can be a small organic molecule in a crystalline phase (matter), or a polymer. Polymer materials can be flexible; such LEDs are known as PLEDs or FLEDs.

Compared with regular LEDs, OLEDs are lighter, and polymer LEDs can have the added benefit of being flexible. Some possible future applications of OLEDs could be:

Inexpensive, flexible displays
Light sources
Wall decorations
Luminous cloth

OLEDs have been used to produce visual displays for portable electronic devices such as cellphones, digital cameras, and MP3 players. Larger displays have been demonstrated, but their life expectancy is still far too short (

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