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Light-emitting diode


 
 



A light-emitting diode (LED) (), is a semiconductorFacts About Semiconductor

A semiconductor is a material with electrical conductivity that is intermediate between that of an insulator and a conducto...
 diodeDiode

In electronics, a diode is a component that restricts the direction of movement of charge carriers....
 that emits light when an electrical current is applied in the forward direction of the device, as in the simple LED circuit. The effect is a form of electroluminescenceElectroluminescence

Electroluminescence is an optical phenomenon and electrical phenomenon where a material emits light in response to an elect...
 where incoherentCoherence (physics)

Coherence is the property of wave-like states that enables them to exhibit interference....
 and narrow-spectrumSpectrum

A spectrum is a condition or value that is not limited to a specific set of values but can vary infinely within a continuum...
 light is emitted from the p-n junctionP-n junction

A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact....
.

LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lightingLighting

Lighting refers to either artificial light sources such as lamps or to natural illumination of interiors from daylight....
.
An LED is usually a small area (less than 1 mm2) light source , often with optics added to the chip to shape its radiation pattern and assist in reflection

.
The colorColor

Color or colour is the visual perceptual property corresponding in humans to the categories called red, y...
 of the emitted light depends on the composition and condition of the semiconducting material used, and can be infraredInfrared

Infrared radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of...
, visibleVisible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye....
, or ultravioletUltraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X...
.






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A light-emitting diode (LED) (), is a semiconductorFacts About Semiconductor

A semiconductor is a material with electrical conductivity that is intermediate between that of an insulator and a conducto...
 diodeDiode

In electronics, a diode is a component that restricts the direction of movement of charge carriers....
 that emits light when an electrical current is applied in the forward direction of the device, as in the simple LED circuit. The effect is a form of electroluminescenceElectroluminescence

Electroluminescence is an optical phenomenon and electrical phenomenon where a material emits light in response to an elect...
 where incoherentCoherence (physics)

Coherence is the property of wave-like states that enables them to exhibit interference....
 and narrow-spectrumSpectrum

A spectrum is a condition or value that is not limited to a specific set of values but can vary infinely within a continuum...
 light is emitted from the p-n junctionP-n junction

A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact....
.

LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lightingLighting

Lighting refers to either artificial light sources such as lamps or to natural illumination of interiors from daylight....
.
An LED is usually a small area (less than 1 mm2) light source , often with optics added to the chip to shape its radiation pattern and assist in reflection

.
The colorColor

Color or colour is the visual perceptual property corresponding in humans to the categories called red, y...
 of the emitted light depends on the composition and condition of the semiconducting material used, and can be infraredInfrared

Infrared radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of...
, visibleVisible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye....
, or ultravioletUltraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X...
. Besides lighting, interesting applications include using UV-LEDs for sterilization of water and disinfection of devices
, and as grow lightGrow light

A grow light is an electric lamp designed to promote plant growth by emitting an electromagnetic spectrum appropriate for ph...
 to enhance photosynthesisFacts About Photosynthesis

Photosynthesis , generally, is the synthesis of sugar from light, carbon dioxide and water, with oxygen as a waste product....
 in plants.

History


Discovery and development

The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. RoundFacts About H. J. Round

Captain Henry Joseph Round was one of the early pioneers of radio and received 117 patents....
 of Marconi Labs. 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,

and no practical use was made of the discovery for several decades.
Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs)Gallium Arsenide

Gallium Arsenide is an independent record label....
 and other semiconductor alloys in 1955

. Braunstein observed infrared emission generated by simple diode structures using GaSb, GaAsGaas

Gaas is a village and commune in the Landes d?partement of south-western France....
, InP, and Ge-Si alloys at room temperature and at 77 K. Experimenters at Texas InstrumentsTexas Instruments

| homepage =}}Texas Instruments , better known in the electronics industry as TI, is an American company ba...
, Bob Biard
and Gary Pittman, found in 1961 that gallium arsenide gave off infrared radiation 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 diodeDiode

In electronics, a diode is a component that restricts the direction of movement of charge carriers....
.

The first practical visible-spectrum (red) LED was developed by Nick Holonyak Jr.Nick Holonyak

Nick Holonyak Jr. was born in Zeigler, Illinois on November 3, 1928....
 in 1962, then of the General Electric Company and later with the University of Illinois at Urbana-ChampaignUniversity of Illinois at Urbana-Champaign Summary

The University of Illinois at Urbana-Champaign, also known as UIUC and the U of I, is the flagship campus in the...
 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 NakamuraShuji Nakamura

Shuji Nakamura is a professor at the University of California, Santa Barbara....
 of Nichia CorporationNichia Corporation Overview

The Nichia Corporation is a Japanese chemical manufacturing company that is most widely known for producing phosphors....
 of Japan demonstrated the first high-brightness blue LED based on InGaNIndium gallium nitride

Indium gallium nitride is a semiconductor material made of a mix of gallium nitride and indium nitride....
, borrowing on critical developments in GaN nucleation on sapphire substrates and the demonstration of p-type doping of GaN which were developed by I. Akasaki and H. Amano in Nagoya. In the 1995 Alberto Barbieri at the Cardiff UniversityCardiff University

Cardiff University is a university in Cardiff....
 Laboratory (GB) investigated the Efficiency and Reliability of high-brightness LED demonstrating very high result by using a transparent contact made by indium tin oxideIndium tin oxide

Indium tin oxide is a mixture of indium oxide and tin oxide, typically 90% In2O3, 10% SnO2 by weight....
 (ITO) on (AlGaInP/GaAs) LED. The existence of the blue LED and high efficiency quickly carried 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 PrizeMillennium Technology Prize

The Millennium Technology Prize is Finland's recognition for innovators that aim to improve quality of life and raise its pr...
 for his invention.

The development of LED technology has caused the efficiency and light output to increase exponentially, with a doubling approximately every 36 month since the 1960's, in a way similar to Moore's lawMoore's Law

Moore's Law is the empirical observation that the transistor density of integrated circuits, with respect to minimum compone...
. The advances are generally attributed to the parallel development of other semiconductor technologies and advances in optics and material science. This trend is normally called Haitz's LawHaitz's Law

Haitz's Law states that every decade, the price of LEDs have fallen by 10x while the performance has increased by 20x, for ...
 after Dr. Roland Haitz.

Practical use


The first commercial LEDs were commonly used as replacements for incandescentIncandescence

Incandescence is the release of electromagnetic radiation from a hot body due to its high temperature....
 indicators, and in seven-segment displaySeven-segment display

A seven-segment display display"), less commonly known as a seven-segment indicator, is a form of display device that ...
s, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level, causing LEDs to become bright enough to be used for illumination.

Most LEDs were made in the very common 5 mm T1ł/4 and 3 mm T1 packages, but with higher power, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages adapted for efficient heat dissipation are becoming common. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs (see, for example, Philips LumiledsPhilips Lumileds Lighting Company Overview

Philips Lumileds is the world's leading manufacturer of high-power light-emitting diodes and a pioneer in the use of solid-s...
).

Used to cause epileptic seizures in small children usually ranging from the ages of 3-12.

LED technology


Physical principles


Like a normal diodeDiode

In electronics, a diode is a component that restricts the direction of movement of charge carriers....
, the LED consists of a chip of semiconducting material impregnated, or dopedFacts About Doping (semiconductor)

In semiconductor production, doping refers to the process of intentionally introducing impurities into an extremely pure sem...
, with impurities to create a p-n junctionP-n junction

A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact....
. As in other diodes, current flows easily from the p-side, or anodeAnode

An anode is the electrode in a device that electrons flow out of to return to the circuit....
, to the n-side, or cathodeCathode

A cathode is the electrode at which electrons go into a cell, tube or diode, whether driven externally or internally....
, but not in the reverse direction. Charge-carriers—electronElectron

The electron is a fundamental subatomic particle that carries an electric charge....
s and holesElectron hole

In solid state physics, an electron hole is the absence of an electron from the otherwise full valence band....
—flow into the junction from electrodeElectrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit ....
s with different voltageVoltage

Voltage is the difference of electrical potential between two points of an electrical network, expressed in volts ....
s. When an electron meets a hole, it falls into a lower energy levelEnergy level

A quantum mechanical system can only be in certain states, so that only certain energy levels are possible....
, and releases energyEnergy

In general, the concept of energy refers to "the potential for causing changes." The word is used in several different conte...
 in the form of a photonFacts About Photon

In modern physics, the photon is the elementary particle responsible for electromagnetic phenomena....
.

The wavelengthWavelength Summary

The wavelength is the distance between repeating units of a wave pattern....
 of the light emitted, and therefore its color, depends on the band gapBand gap

In solid state physics and related applied fields, the band gap is the energy difference between the top of the valence band...
 energy of the materials forming the p-n junction. In siliconSilicon

Silicon is the chemical element in the periodic table that has the symbol Si and atomic number 14....
 or germaniumGermanium

.Germanium is a chemical element in the periodic table that has the symbol Ge and atomic number 32....
 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 the 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 arsenideGallium Arsenide Summary

Gallium Arsenide is an independent record label....
. Advances in materials scienceMaterials science

Materials science is a multi-disciplinary field involving the properties of matter and its applications to various areas of s...
 have made possible the production of devices with ever-shorter wavelengthWavelength

The wavelength is the distance between repeating units of a wave pattern....
s, 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 sapphireSapphire

Sapphire is the single-crystal form of aluminium oxide , a mineral known as corundum....
 substrate.

Light extraction

Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, and light spreading layer, increase the LED efficiency. The refractive indexRefractive index

The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed in that...
 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 coefficientsFresnel equations

The Fresnel equations, deduced by Augustin-Jean Fresnel, describe the behaviour of light when moving between media of differ...
), 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, by introducing random roughness, creating programmed moth eye surface patterns. Recently Photonic crystalPhotonic crystal

Photonic crystals are periodic dielectric or metallo-dielectricstructures that are designed to affect the propagation of ele...
 have also been used to minimize back-reflections
. In December 2007, scientists at Glasgow University claimed to have found a way to make Light Emitting Diodes brighter and use less power than energy efficient light bulbs currently on the market by imprinting holes into billions of LEDs in a new and cost effective method using a process known as nanoimprint lithographyNanoimprint Lithography

Nanoimprint lithography is a novel method of fabricating nanometer scale patterns....
.

Materials

Conventional LEDs are made from a variety of inorganic semiconductor materialsSemiconductor materials

Semiconductor materials are insulators at absolute zero temperature that conduct electricity in a limited way at room temper...
, producing the following colors:

  • Aluminium gallium arsenideAluminium gallium arsenide

    Aluminium gallium arsenide is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger band...
     (AlGaAs) — red and infraredInfrared

    Infrared radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of...
  • Aluminium gallium phosphideAluminium gallium phosphide

    Aluminium gallium phosphide, a phosphide of aluminium and gallium, is a semiconductor material....
     (AlGaP) — green
  • Aluminium gallium indium phosphideAluminium gallium indium phosphide

    Aluminium gallium indium phosphide is a semiconductor material....
     (AlGaInP) — high-brightness orange-red, orange, yellow, and green
  • Gallium arsenide phosphideGallium arsenide phosphide

    Gallium arsenide phosphide is a semiconductor material, an alloy of gallium arsenide and gallium phosphide....
     (GaAsP) — red, orange-red, orange, and yellowYellow

    Yellow is any color of light that stimulates both the red and green cone cells of the retina, but not the blue cone cells....
  • Gallium phosphide (GaP) — red, yellow and green
  • Gallium nitride (GaN) — green, pure green (or emerald green), and blueBlue

    Blue is any of a number of similar colors....
     also white (if it has an AlGaN Quantum Barrier)
  • Indium gallium nitrideIndium gallium nitride

    Indium gallium nitride is a semiconductor material made of a mix of gallium nitride and indium nitride....
     (InGaN) — 450–470 nm — near ultraviolet, bluish-green and blue
  • Silicon carbideSilicon carbide

    | Silicon carbide|-| align="center" colspan="2" | |-...
     (SiC) as substrate — blue
  • SiliconSilicon Summary

    Silicon is the chemical element in the periodic table that has the symbol Si and atomic number 14....
     (Si) as substrate — blue (under development)
  • SapphireSapphire Summary

    Sapphire is the single-crystal form of aluminium oxide , a mineral known as corundum....
     (Al2O3) as substrate — blue
  • Zinc selenideZinc selenide

    Zinc selenide, is a light yellow binary solid compound....
     (ZnSe) — blue
  • DiamondDiamond

    Diamond is the hardest known natural material and one of the two best known forms of carbon, whose hardness and high disper...
     (C) — ultraviolet
  • Aluminium nitrideAluminium nitride

    Aluminium nitride is a nitride of aluminium....
     (AlN), aluminium gallium nitrideAluminium gallium nitride

    Aluminium gallium nitride is a semiconductor material....
     (AlGaN), aluminium gallium indium nitride (AlGaInN) — near to far ultravioletFacts About Ultraviolet

    Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X...
     (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




Blue LEDs are based on the wide band gapBand gap

In solid state physics and related applied fields, the band gap is the energy difference between the top of the valence band...
 semiconductors GaN and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of whiteWhite

White is a color that has high brightness but zero hue....
 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 LaboratoriesRCA

RCA, formerly an initialism for the Radio Corporation of America, is now a trademark used by two companies for product...
. However, these devices had too little light output to be of much practical use. In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping by Isamu AkasakiIsamu Akasaki

, is a Japanese scientist, best known for inventing p-n junction blue LEDs using gallium nitride in as early as 1989, first in th...
 and Hiroshi Amano (Nagoya, Japan) ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high brightness blue LEDs were demonstrated through the work of Shuji NakamuraShuji Nakamura

Shuji Nakamura is a professor at the University of California, Santa Barbara....
 at Nichia CorporationNichia Corporation

The Nichia Corporation is a Japanese chemical manufacturing company that is most widely known for producing phosphors....
.

By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN quantum wellQuantum well

A quantum well is a potential well that confines particles, which were originally free to move in three dimensions, to two d...
s 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 aluminium gallium nitrideAluminium gallium nitride

Aluminium gallium nitride is a semiconductor material....
 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 nitrides containing aluminium, most often AlGaNAluminium gallium nitride Overview

Aluminium gallium nitride is a semiconductor material....
 and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often encountered, for example, as black lightBlack light

Black light is the common name for a lamp emitting electromagnetic radiation that is almost exclusively in the soft near ult...
 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. As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNADNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions for the biological development of a cellu...
, with a peak at about 260 nm, UV LEDs emitting at 250–270 nm are to be 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.

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

Aluminium nitride is a nitride of aluminium....
.

While not 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 light LEDs


There are two ways of producing high intensity whiteWhite

White is a color that has high brightness but zero hue....
-light using LEDs. One is to use individual LEDs that emit three primary colorPrimary color

A primary color or colour is a color that cannot be created by mixing other colors in the gamut of a given color space...
s – red, green, and blue, and then mix all the colors to produce whiteWhite

White is a color that has high brightness but zero hue....
 light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light.
RGB Systems



White lightWhite Light

White Light may refer to:*a blend of all the colors in the visual spectrum, resulting in the color White...
 can be produced by mixing differently colored light, the most common method is to use red, green and blue (RGB). Hence the method is called multi-colored white LEDs (sometimes referred to as RGB LEDs). Because its mechanism is involved with sophisticated electro-optical design to control the blending and diffusionDiffusion

Diffusion, being the spontaneous spreading of matter , heat, or momentum, is one type of transport phenomenon....
 of different colors, this approach has rarely been used to mass produce white LEDs in the industry. Nevertheless this method is particularly interesting to many researchers and scientists because of the flexibility of mixing different colors. In principle, this mechanism also has higher quantum efficiency in producing white light.

There are several types of multi-colored white LEDs: di-, tri-Tri-

Tri- or tria- is a numerical prefix meaning 3 that can be either Greek or Latin; though occasionally tre- and ter- als...
, and tetrachromatic white LEDs. Several key factors that play among these different approaches include color stabilityStability

Stability can refer to:*Ecological stability, measure of the probability of a population returning quickly to a previous st...
, color rendering capability, and luminous efficacyLuminous efficacy

Luminous efficacy is a property of light sources, which indicates what portion of the emitted electromagnetic radiation is u...
. Often higher efficacy will mean lower color rendering, presenting a trade off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficiency (120 lm/W), but the lowest color rendering capability. Oppositely although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous efficiency (>70 lm/W) and fair color rendering capability.

What multi-color LEDs offer is not merely another solution of producing white light, but is a whole new technique of producing light of different colors. In principle, all perceivable colors in the can be produced by mixing different amount of three primary colors, and this makes it possible to produce precise dynamic color control as well. As more effort is devoted to investigating this technique, multi-color LEDs should have profound influence on the fundamental method which we use to produce and control light color. However, before this type of LED can truly play a role on the market, several technical problems need to be solved. These certainly include that this type of LED's emission power decays exponentially with increasing temperature,
resulting in a substantial change in color stability. Such problem is not acceptable for industrial usage. Therefore, many new package designs aiming to solve this problem have been proposed, and their results are being reproduced by researchers and scientists.
Phosphor based LEDs



The method involves coatingCoating

A coating is a covering that is applied to an object to protect it or change its appearance....
 a LED of one color (mostly blue LED made of InGaN) with phosphorPhosphor

A phosphor is a substance that exhibits the phenomenon of phosphorescence....
 of a different color to produce white light. Depending on the color of the original LED, phosphors of different colors can also be employed. By applying several phosphor layers of distinct colors, we can effectively increase the color rendering index (CRI) value of a given LED.

Because this method of producing white LEDs heavily employs the usage of phosphor, the resultant LEDs are called phosphor based white LEDs. Although easier to be manufactured than multi-colored LEDs, phosphor based LEDs have a lower quantum efficiency and other phosphor-related degradation issues. However, it is still the most popular technique of manufacturing high intensity white LEDs as well as high intensity LEDs of other colors because it requires much easier material processing and therefore suits today’s applications. Much effort has been spent on optimizing the operating environment, namely temperature and current, for this type of LED.

Phosphor based white LEDs encapsulate InGaN blue LEDs inside of a phosphor coated epoxy. A common yellow phosphor material is ceriumCerium

Cerium is a chemical element in the periodic table that has the symbol Ce and atomic number 58. ...
-doped yttrium aluminum garnet (Ce3+:YAG). Although the phosphor based white LEDs have a relatively easier mechanism, they reach the fundamental limitation due to the unavoidable Stokes shiftStokes shift

Stokes shift is the difference between positions of the band maxima of the absorption and luminescence spectra of the same e...
  energy loss, a loss that occurs when short wavelength photons are converted to long wavelengthWavelength

The wavelength is the distance between repeating units of a wave pattern....
 photons. Regardless this technique of manufacturing is adopted by most of the LED industry because of its low cost and high output. All the high intensity white LEDs now on the market are manufactured by this method.

Phosphor based white LEDs is so far the simplest solution to produce high intensity white light. With its simplified mechanism, this type of LEDs has attracted much interest from the lighting industry. Because of their more stable performance over a range of temperatures, prototypes as well as products based on this phosphor based mechanism have already appeared on the market. And more high intensity white LEDs are expected to be produced in the near future. However, the biggest challenge these phosphorFacts About Phosphor

A phosphor is a substance that exhibits the phenomenon of phosphorescence....
 based white LEDs face is solving the seemingly unavoidable StokesStokes Overview

Stokes is a surname, and may refer to:...
 energy loss. Again this can be done by adapting a better package design or by replacing a more suitable type of phosphor. Philips LumiledsPhilips Lumileds Lighting Company

Philips Lumileds is the world's leading manufacturer of high-power light-emitting diodes and a pioneer in the use of solid-s...
 patented conformal coating process addresses the issue of varying phosphor thickness, giving the white LEDs a more consistent spectrum of white light.

White LEDs can also be made by coatingCoating

A coating is a covering that is applied to an object to protect it or change its appearance....
 near ultravioletUltraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X...
 (NUV) emitting LEDs with a mixture of high efficiency europiumEuropium

Europium is a chemical element in the periodic table that has the symbol Eu and atomic number 63....
-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 lampFluorescent lamp

A fluorescent lamp is a gas-discharge lamp that uses electricity to excite mercury vapor in argon or neon gas, resulting in ...
s work. However, the ultravioletUltraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X...
 light causes photodegradationPhotodegradation

Photodegradation is degradation of a photodegradable molecule caused by the absorption of photons, particularly those ...
 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 shiftStokes shift

Stokes shift is the difference between positions of the band maxima of the absorption and luminescence spectra of the same e...
 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 grown zinc selenideZinc selenide

Zinc selenide, is a light yellow binary solid compound....
 (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate

Quantum Dot LEDs


A new technique developed by Michael Bowers, a graduate student at Vanderbilt UniversityVanderbilt University

Vanderbilt University is a private, nonsectarian, coeducational research university in Nashville, Tennessee....
 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.

Quantum Dots are semiconductorSemiconductor

A semiconductor is a material with electrical conductivity that is intermediate between that of an insulator and a conducto...
 nanocrystals that possess unique optical properties. Their emission color can be tuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on the CIEInternational Commission on Illumination

The International Commission on Illumination is the international authority on light, illumination, colour, and colour space...
 diagram. This provides more color options and better color rendering white LEDs. Quantum dot LEDs are available in the same package types as traditional phosphorFacts About Phosphor

A phosphor is a substance that exhibits the phenomenon of phosphorescence....
 based LEDs.

Organic light-emitting diodes (OLEDs)



If the emitting layer material of the LED is an organic compoundOrganic compound

An organic compound is any member of a large class of chemical compounds whose molecules contain carbon and hydrogen; theref...
, it is known as an Organic Light Emitting Diode. To function as a semiconductor, the organic emitting material must have conjugated pi bondsConjugated system

A chemically conjugated system is a system of atoms covalently bonded with alternating single and multiple bonds in a molecu...
. The emitting material can be a small organic moleculeMolecule

In chemistry, a molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds....
 in a crystalCrystal

In chemistry and mineralogy, a crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regular...
line phasePhase (matter)

In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical ...
, or a polymerPolymer

Polymer is a term used to describe molecules consisting of structural units and a large number of repeating units connected ...
. 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 (<1,000 hours) to be practical.

Today, OLEDs operate at substantially lower efficiency than inorganic (crystalline) LEDs. The best luminous efficacy of an OLED so far is about 68 lm/W .

Efficiency and operational parameters


Most typical LEDs are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. Around 1999, Philips LumiledsPhilips Lumileds Lighting Company

Philips Lumileds is the world's leading manufacturer of high-power light-emitting diodes and a pioneer in the use of solid-s...
 introduced power LEDs capable of continuous use at one wattWatt

The watt is the SI derived unit of power, equal to one joule per second....
 (W). These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for heat removal from the LED die.

One of the key advantages of LED-based lighting is its high efficiency, as measured by its light output per unit power input. White LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems. In 2002, LumiledsPhilips Lumileds Lighting Company

Philips Lumileds is the world's leading manufacturer of high-power light-emitting diodes and a pioneer in the use of solid-s...
 made five-watt LEDs available with a luminous efficacyLuminous efficacy

Luminous efficacy is a property of light sources, which indicates what portion of the emitted electromagnetic radiation is u...
 of 18–22 lumensLumen (unit)

The lumen is the SI unit of luminous flux....
 per watt (lm/W). For comparison, a conventional 60–100 W incandescent lightbulb produces around 15 lm/W, and standard fluorescent lights produce up to 100 lm/W. (The luminous efficacyLuminous efficacy

Luminous efficacy is a property of light sources, which indicates what portion of the emitted electromagnetic radiation is u...
 article discusses these comparisons in more detail.)

In September 2003, a new type of blue LED was demonstrated by the company Cree, Inc.Cree Inc.

Cree Inc. is a Durham, North Carolina based, American corporation which manufactures semiconductor materials and devices....
 to provide 24 mW at 20 milliamperes (mA). This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006 they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Also, Seoul SemiconductorSeoul Semiconductor

Seoul Semiconductor Co., Ltd is a Korean company manufacturing LED devices....
 has plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be approaching an order of magnitude improvement over standard incandescents and better even than standard fluorescents. Nichia CorporationNichia Corporation

The Nichia Corporation is a Japanese chemical manufacturing company that is most widely known for producing phosphors....
 has developed a white light LED with luminous efficacy of 150 lm/W at a forward current of 20 mA. In May 2008, 130lm/W is available from Chinese LED manufacturers.

It should be noted that high-power (= 1 W) LEDs are necessary for practical general lighting applications. Typical operating currents for these devices begin at 350 mA. The highest efficiency high-power white LED is claimed by Philips Lumileds Lighting Co. with a luminous efficacy of 115 lm/W (350 mA).

Failure modes


The most common way for LEDs (and diode lasers) to fail is the gradual lowering of light output and loss of efficiency. However, sudden failures can occur as well.

The mechanism of degradation of the active region, where the radiative recombination occurs, involves nucleationNucleation

Nucleation is the onset of a phase transition in a small region....
 and growth of dislocationDislocation

In materials science, a dislocation is a linear crystallographic defect, or irregularity, within a crystal structure....
s; this requires a presence of an existing defect in the crystal and is accelerated by heat, high current density, and emitted light. Gallium arsenideGallium Arsenide

Gallium Arsenide is an independent record label....
 and aluminium gallium arsenideAluminium gallium arsenide

Aluminium gallium arsenide is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger band...
 are more susceptible to this mechanism than gallium arsenide phosphideFacts About Gallium arsenide phosphide

Gallium arsenide phosphide is a semiconductor material, an alloy of gallium arsenide and gallium phosphide....
 and indium phosphide. Due to different properties of the active regions, gallium nitride and indium gallium nitrideIndium gallium nitride

Indium gallium nitride is a semiconductor material made of a mix of gallium nitride and indium nitride....
 are virtually insensitive to this kind of defect; however, high current density can cause electromigrationElectromigration

Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum ...
 of atoms out of the active regions, leading to emergence of dislocations and point defects, acting as nonradiative recombination centers and producing heat instead of light. Ionizing radiationIonizing radiation

Ionizing radiation has many practical uses, but it is also dangerous to human health....
 can lead to the creation of such defects as well, which leads to issues with radiation hardeningRadiation hardening

Microelectronics designed for environments with high levels of ionizing radiation have special design challenges....
 of circuits containing LEDs (e.g., in optoisolators). Early red LEDs were notable for their short lifetime.

White LEDs often use one or more phosphorPhosphor

A phosphor is a substance that exhibits the phenomenon of phosphorescence....
s. The phosphors tend to degrade with heat and age, losing efficiency and causing changes in the produced light color. Pink LEDs often use an organic phosphor formulation which may degrade after just a few hours of operation causing a major shift in output color.

High electrical currents or voltages at elevated temperatures can cause diffusion of metal atoms from the electrodes into the active region. Some materials, notably indium tin oxideIndium tin oxide

Indium tin oxide is a mixture of indium oxide and tin oxide, typically 90% In2O3, 10% SnO2 by weight....
 and silverSilver

Silver is a chemical element with the symbol Ag ....
, are subject to electromigration with the conseguence of leakage current and non radiative recombination along the chip edges. In some cases, especially with GaN/InGaN diodes, a barrier metalBarrier metal

A barrier metal is a material used in integrated circuits to chemically isolate semiconductors from soft metal interconnects...
 layer is used to hinder the electromigration effects. Mechanical stresses, high currents, and corrosive environment can lead to formation of whiskersWhisker (metallurgy)

Metal whiskers are a crystalline metallurgical phenomenon whereby metal grows tiny, filiform hairs....
, causing short circuits.

High-power LEDs are susceptible to current crowdingCurrent crowding

Current crowding is a nonhomogenous distribution of current density through a conductor or semiconductor, especially at the ...
, nonhomogenous distribution of the current density over the junction. This may lead to creation of localized hot spots, which poses risk of thermal runawayThermal runaway

Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further...
. Nonhomogenities in the substrate, causing localized loss of thermal conductivityThermal conductivity Summary

In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat....
, aggravate the situation; most common ones are voids caused by incomplete solderingFacts About Soldering

Soldering is a method of joining metal parts using an alloy of low melting point filler material which has a melting temper...
, or by electromigration effects and Kirkendall voiding. Thermal runaway is a common cause of LED failures.

Laser diodeLaser diode Summary

A laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode....
s may be subject to catastrophic optical damageCatastrophic optical damage

Catastrophic optical damage is a failure mode of high-power semiconductor lasers....
, when the light output exceeds a critical level and causes melting of the facet.

Some materials of the plastic package tend to yellow when subjected to heat, causing partial absorption (and therefore loss of efficiency) of the affected wavelengths.

Sudden failures are most often caused by thermal stresses. When the epoxy resin used in packaging reaches its glass transition temperatureGlass transition temperature

A materials glass transition temperature, Tg, is the temperature below which molecules have little relative mobility....
, it starts rapidly expanding, causing mechanical stresses on the semiconductor and the bondedFacts About Wire bonding

Wire bonding is a method of making interconnections between a microchip and the outside world as part of semiconductor devic...
 contact, weakening it or even tearing it off. Conversely, very low temperatures can cause cracking of the packaging.

Electrostatic dischargeElectrostatic discharge

Electrostatic discharge is the sudden and momentary electric current that flows when an excess of electric charge, stored on...
 (ESD) may cause immediate failure of the semiconductor junction, a permanent shift of its parameters, or latent damage causing increased rate of degradation. LEDs and lasers grown on sapphireSapphire Overview

Sapphire is the single-crystal form of aluminium oxide , a mineral known as corundum....
 substrate are more susceptible to ESD damage.

Research on DNA

The DNA in salmon sperm has recently been discovered to amplify the effects and quality of a LED light.

See research at University of Cincinnati: http://www.uc.edu/news/NR.asp?id=7089

Articles from the UK:
http://www.theregister.co.uk/2007/09/18/led_salmon_dna_product_enhancement/

Considerations in use





Unlike incandescent light bulbIncandescent light bulb

The incandescent light bulb or incandescent lamp is a source of artificial light that works by incandescence....
s, which light up regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating currentFacts About Alternating current

An alternating current is an electrical current whose magnitude and direction vary cyclically, as opposed to direct current...
 voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.

While the only definitive way to determine the polarity of the LED is to examine its datasheet, these methods are usually reliable:
sign:+-
terminal:anode (A)cathode (K)
leads:longshort
exterior:roundflat
interior:smalllarge
wiring:redblack


Less reliable methods of determining polarity are:
sign:+-
marking:nonestripe
pin:12
PCBPrinted circuit board

In electronics, printed circuit boards, or PCBs, are used to mechanically support and electrically connect elect...
:
roundsquare


While it is not an officially reliable method, it is almost universally true that the cup that holds the LED die corresponds to the cathode. It is strongly recommended to apply a safe voltage and observe the illumination as a test regardless of what method is used to determine the polarity.

Because the voltage versus current characteristics of the LED are much like any diodeDiode

In electronics, a diode is a component that restricts the direction of movement of charge carriers....
 (that is, current approximately an exponential function of voltage), a small voltage change results in a huge change in current. Added to deviations in the process this means that a voltage sourceVoltage source

A voltage source is any device or system that produces an electromotive force between its terminals OR derives a se...
 may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it.

Since the voltage is logarithmically related to the current it can be considered to remain largely constant over the LED's operating range. Thus the power can be considered to be essentially proportional to the current. In order to keep power nearly constant with variations in supply and LED characteristics, the power supply should be a “current sourceCurrent source

A current source is an electrical or electronic device that delivers or absorbs electric current....
”, that is, it should supply an almost constant current. If high efficiency is not required (e.g., in most indicator applications), an approximation to a current source is made by connecting the LED in series with a current limiting resistor to a regulated voltage source.

Most LEDs have low reverse breakdown voltageBreakdown voltage

Breakdown Voltage = The minimum voltage that makes an insulator react as a conductor....
 ratings, so they will also be damaged by an applied reverse voltage of more than a few volts. Since some manufacturers don't follow the indicator standards above, if possible the data sheet should be consulted before hooking up the LED, or the LED may be tested in series with a resistorResistor

|- align = "center"||width = "25"|| |- align = "center"...
 on a sufficiently low voltage supply to avoid the reverse breakdown. If it is desired to drive the LED directly from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in inverse parallelAntiparallel (electronics) Summary

In electronics, two antiparallel or inverse-parallel devices are connected in parallel but with their polarities rever...
.

LEDs can be purchased with built in series resistors. These can save PCBPrinted circuit board

In electronics, printed circuit boards, or PCBs, are used to mechanically support and electrically connect elect...
 space and are especially useful when building prototypePrototype

A prototype is an original type, form, or instance of some thing serving as a typical example, basis, epitome, or standard f...
s or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. To increase efficiency (or to allow intensity control without the complexity of a DACDigital-to-analog converter

In electronics, a digital-to-analog converter is a device for converting a digital code to an analog signal....
), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion thresholdFlicker fusion threshold

The flicker fusion threshold is a concept in the psychophysics of vision....
, the LED will appear to be continuously lit.

Multiple LEDs can be connected in seriesFacts About Series and parallel circuits

Series and parallel electrical circuits are two basic ways of wiring components....
 with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. ParallelSeries and parallel circuits

Series and parallel electrical circuits are two basic ways of wiring components....
 operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.

Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically redRed

Red is any of a number of similar colors at the lowest frequencies of light discernible by the human eye....
 and greenGreen

Green is a color with many different shades, all within a wavelength of roughly 520570 nm....
), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity, however, more than two electrodes (leads) are required.

LEDs are usually constantly illuminated when a current passes through them, but flashing LEDs are also available. Flashing LEDs resemble standard LEDs but they contain an integrated multivibratorMultivibrator

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers...
 circuit inside which causes the LED to flash with a typical period of one second. This type of LED comes most commonly as red, yellow, or green. Most flashing LEDs emit light of a single wavelength, but multicolored flashing LEDs are available too.

Generally, for newer common standard LEDs in 3 mm or 5 mm packages, the following forward DC potential differences are typically measured. The forward potential difference depending on the LED's chemistry, temperature, and on the current (values here are for approx. 20 mA, a commonly-found maximum value).

Color Potential Difference (Vf)
Infrared 1.6 V
Red 1.8–2.1 V
Orange 2.2 V
Yellow 2.4 V
Green 2.6 V
Blue 3.0–3.5 V
White 3.0–3.5 V
Ultraviolet 3.5 V


Many LEDs are rated at 3 V maximum reverse potential.

LEDs also behave as photocells, and will generate a current depending on the ambient light. They are not efficient as photocells, and will only produce a few microamperes (µA), but will produce a electrical potential—as much as 2 or 3 V depending on the band gapBand gap Overview

In solid state physics and related applied fields, the band gap is the energy difference between the top of the valence band...
. This is enough to operate an amplifier or a CMOS logic gate. This effect can be used to make an inexpensive light sensor, for example to decide when to turn on the LED illuminator.

Advantages of using LEDs




  • LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.
  • LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
  • 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.
  • When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
  • LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
  • LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
  • 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 longer. Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.
  • LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
  • LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states “less than 100ns.” LEDs used in communications devices can have even faster response times.
  • LEDs can be very small and are easily populated onto printed circuit boards.
  • LEDs do not contain mercuryMercury (element)

    Mercury, also called quicksilver, is a chemical element in the periodic table that has the symbol Hg and atomic...
    , unlike compact fluorescent lampCompact fluorescent lamp

    A compact fluorescent lamp, also known as a compact fluorescent light bulb or an energy saving lightbulb, is a t...
    s.
  • Due to the human eye's visual persistence LEDs can be pulse width or duty cycle modulated in order to save power or achieve an apparent higher brightness for a given power input. The eye will tend to perceive the peak current light level rather than the average current light level when the modulation rate is higher than approximately 1000 hertz and the duty cycle is greater than 15 to 20%. This is also useful when applied to the multiplexing used in 7-segment displays.



Disadvantages of using LEDs


  • LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps.
  • LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinkingHeat sink

    A heat sink is an environment or object that absorbs and dissipates heat from another object using thermal contact....
     is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
  • LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.
  • The spectrum of some white LEDs differs significantly from a black bodyBlack body

    In physics, a black body is an object that absorbs all electromagnetic radiation that falls onto it....
     radiator, such as the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differentlyColor vision

    Color vision is the capacity of an organism or machine to distinguish objects based on the wavelength of the light they refl...
     under LED illumination than sunlight or incandescent sources, due to metamerismMetamerism (color)

    Metamerism is the situation where two color samples with different spectral power distributions appear to be the same color ...
    . Color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
  • LEDs do not approximate a “point source” of light, so cannot be used in applications needing a highly collimated beam. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.
  • There is increasing concern that blue LEDs and white LEDs are now capable of exceeding safe limits of the so-called blue-light hazardFacts About Blue-light hazard

    Blue-light hazard is defined as the potential for a photochemical induced retinal injury resulting from radiation exposure a...
     as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.

Types

There are three main types of LEDs: miniature, alphanumeric, and illumination.

Miniature LEDs

These are mostly single-die LEDs used as indicators, and they come in various-size packages:
  • surface mount
  • 2 mm
  • 3 mm (T1)
  • 5 mm (T1ł/4)
  • 10 mm
  • Other sizes are also available, but less common.


Common package shapes:
  • Round, dome top
  • Round, flat top
  • Rectangular, flat top (often seen in LED bar-graph displays)
  • Triangular or square, flat top


The encapsulation may also be clear or semi opaque to improve contrast and viewing angle.

There are three main categories of miniature single die LEDs:
  • Low current — typically rated for 2 mA at around 2 V (approximately 4 mW consumption).
  • Standard — 20 mA LEDs at around 2 V (approximately 40 mW) for red, orange, yellow & green, and 20 mA at 4–5 V (approximately 100 mW) for blue, violet and white.
  • Ultra-high output — 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.

Multi-color LEDs
A “bi-color LED” is actually two different LEDs in one case. It consists of two dies connected to the same two leads but in opposite directions. Current flow in one direction produces one color, and current in the other direction produces the another color. Alternating the two colors with sufficient frequency causes the appearance of a third color.

A “tri-color LED” is also two LEDs in one case, but the two LEDs are connected to separate leads so that the two LEDs can be controlled independently and lit simultaneously.

RGB LEDs contain red, green and blue emitters, generally using a four-wire connection with one common (anode or cathode).
Five- and twelve-volt LEDs
These are miniature LEDs incorporating a series resistorResistor

|- align = "center"||width = "25"|| |- align = "center"...
, and may be connected directly to a 5 V or 12 V supply.
Flashing LEDs
These miniature LEDs flash when connected to 5 V or 12 V. Used as attention seeking indicators where it is desired to avoid the complexity of external electronics.

Alphanumeric LEDs

LED displays are available in seven-segmentSeven-segment display

A seven-segment display display"), less commonly known as a seven-segment indicator, is a form of display device that ...
 and starburst format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters.

Seven-segment LED displays were in widespread use in the 1970s and 1980s, but increasing use of liquid crystal displayLiquid crystal display

A liquid crystal display is a thin, flat display device made up of any number of color or monochrome pixels arrayed in fron...
s, with their lower power consumption and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

Lighting LEDs

LED lampLED lamp

A LED lamp is a type of solid state lighting that utilizes light-emitting diodes as a source of illumination rather than e...
s (also called LED bars or Illuminators) are usually clusters of LEDs in a suitable housing. They come in different shapes, among them the light bulb shape with a large E27 Edison screwEdison screw Overview

The Edison screw fitting is a system of light bulb connectors, developed by Thomas Edison in 1909 under the Mazda trademark....
 and MR16MR16

MR16 is a standard format for halogen reflector lamps made by a variety of manufactures....
 shape with a bi-pin base. Other models might have a small Edison E14Edison screw

The Edison screw fitting is a system of light bulb connectors, developed by Thomas Edison in 1909 under the Mazda trademark....
 fitting, GU5.3 (Bipin cap) or GU10Bayonet mount

A bayonet mount or bayonet connector is a fastening mechanism that relies on mated surfaces; a male side with one or m...
 (bayonet socket). This includes low-voltage (typically 12 V halogen-like) varieties and replacements for regular AC mains (120-240 V AC) lighting. Currently the latter are less widely available but this is changing rapidly.

Seoul Semiconductor Co., LtdSeoul Semiconductor Summary

Seoul Semiconductor Co., Ltd is a Korean company manufacturing LED devices....
 produces LEDs that can run directly from mains power without the need for a DC converter. For each half cycle part of the LED diode emits light and part is dark, and this is reversed during the next half cycle. Current efficiency is 80 lm/W.

LED applications

















List of LED applications

Some of these applications are further elaborated upon in the following text.
Devices, medical applications, clothing, toys