02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #1

Light�emitting diodeFrom Wikipedia, the free encyclopedia

A light�emitting�diode (LED) (pronounced /ˌɛliːˈdiː/),[1] is asemiconductor diode that emits light when an electric current is appliedin the forward direction of the device, as in the simple LED circuit. Theeffect is a form of electroluminescence where incoherent andnarrow�spectrum light is emitted from the p�n junction in a solid statematerial.

LEDs are widely used as indicator lights on electronic devices andincreasingly in higher power applications such as flashlights and arealighting. An LED is usually a small area (less than 1 mm2) light source,often with optics added directly on top of the chip to shape its radiationpattern and assist in reflection.[2][3] The color of the emitted lightdepends on the composition and condition of the semiconductingmaterial used, and can be infrared, visible, or ultraviolet. Besideslighting, interesting applications include using UV�LEDs for sterilization ofwater and disinfection of devices,[4] and as a grow light to enhancephotosynthesis in plants.[5]

Contents1 History

1.1 Discoveries and early devices1.2 Practical use1.3 Continuing development

2 Technology2.1 Light extraction2.2 Efficiency and operational parameters2.3 Electrical polarity2.4 Failure modes

3 Colors and materials3.1 Ultraviolet and blue LEDs3.2 White light LEDs

3.2.1 RGB Systems3.2.2 Phosphor based LEDs

3.3 Organic light�emitting diodes (OLEDs)3.4 Quantum Dot LEDs (experimental)

4 Types4.1 Miniature LEDs4.2 Flashing LEDs4.3 High power LEDs4.4 Multi�color LEDs4.5 Alphanumeric LED displays

5 Considerations for use5.1 Power sources5.2 Lighting LEDs on mains5.3 Advantages of using LEDs5.4 Disadvantages of using LEDs

6 Applications6.1 Indicators and signs6.2 Lighting6.3 Smart lighting6.4 Non�visual applications6.5 Light sources for machine vision systems

7 Examples of use7.1 Christmas lights

8 See also9 References10 External links

HistoryDiscoveries and early devices

The first known report of a light�emitting solid�state diode was made in 1907 by the British experimenter H. J. Roundof Marconi Labs when he noticed electroluminescence produced from a crystal of silicon carbide while using acat's�whisker detector.[6] Russian Oleg Vladimirovich Losev independently created the first LED in the mid 1920s; hisresearch, though distributed in Russian, German and British scientific journals, was ignored,[7][8] and no practical usewas made of the discovery for several decades. Rubin Braunstein of the Radio Corporation of America reported oninfrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955.[9] Braunstein observedinfrared emission generated by simple diode structures using GaSb, GaAs, InP, and Ge�Si alloys at room temperatureand at 77 kelvin.

In 1961, experimenters Bob Biard and Gary Pittman working at Texas Instruments,[10] found that gallium arsenidegave off infrared radiation when electric current was applied. Biard and Pittman were able to establish the priority oftheir work and received the patent for the infrared light�emitting diode.

LED schematic symbol

LED displays allow for smaller sets ofinterchangeable LEDs to be one large display.

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #2The first practical visible�spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at GeneralElectric Company. He later moved to the University of Illinois at Urbana�Champaign.[11] Holonyak is seen as the "fatherof the light�emitting diode".[12] M. George Craford, a former graduate student of Holonyak's, invented the first yellowLED and 10x brighter red and red�orange LEDs in 1972.[13] Up to 1968 visible and infrared LEDs were extremelycostly, on the order of US $200 per unit, and so had little practical application. [14] The Monsanto Corporation was thefirst organization to mass�produce visible LEDs, using gallium arsenide phosphide in 1968 to produce red LEDssuitable for indicators. [15] Hewlett Packard (HP) introduced light�emitting diodes in 1968, initially using GaAsPmaterial supplied by Monsanto. The technology proved to have major applications for alphanumeric displays and wasintegrated into HP’s early handheld calculators.

Practical use

The first commercial LEDs were commonly used as replacements for incandescentindicators, and in seven�segment displays, first in expensive equipment such aslaboratory and electronics test equipment, then later in such appliances as TVs,radios, telephones, calculators, and even watches (see list of signal applications).These red LEDs were bright enough only for use as indicators, as the light outputwas not enough to illuminate an area. Later, other colors became widely availableand also appeared in appliances and equipment. As the LED materials technologybecame more advanced, the light output was increased, while maintaining theefficiency and the reliability to an acceptable level. The invention and developmentof the high power white light LED led to use for illumination (see list of illuminationapplications).

Most LEDs were made in the very common 5 mm T1¾ and 3 mm T1 packages, butwith increasing power output, it has become increasingly necessary to shed excess heat in order to maintainreliability, so more complex packages have been adapted for efficient heat dissipation. Packages for state�of�the�arthigh power LEDs bear little resemblance to early LEDs.

Continuing development

The first high�brightness blue LED was demonstrated by Shuji Nakamura of Nichia Corporation and was based onInGaN borrowing on critical developments in GaN nucleation on sapphire substrates and the demonstration of p�typedoping of GaN which were developed by Isamu Akasaki and H. Amano in Nagoya. In 1995, Alberto Barbieri at theCardiff University Laboratory (GB) investigated the efficiency and reliability of high�brightness LEDs demonstrated avery impressive result by using a transparent contact made of indium tin oxide (ITO) on (AlGaInP/GaAs) LED. Theexistence of blue LEDs and high efficiency LEDs quickly led to the development of the first white LED, which employeda Y3Al5O12:Ce, or "YAG", phosphor coating to mix yellow (down�converted) light with blue to produce light thatappears white. Nakamura was awarded the 2006 Millennium Technology Prize for his invention.[16]

The development of LED technology has caused their efficiency and light output to increase exponentially, with adoubling occurring about every 36 months since the 1960s, in a similar way to Moore's law. The advances aregenerally attributed to the parallel development of other semiconductor technologies and advances in optics andmaterial science. This trend is normally called Haitz's Law after Dr. Roland Haitz.

TechnologyLike a normal diode, the LED consists of a chip ofsemiconducting material impregnated, or doped, with impuritiesto create a p�n junction. As in other diodes, current flows easilyfrom the p�side, or anode, to the n�side, or cathode, but not inthe reverse direction. Charge�carriers—electrons and holes—flowinto the junction from electrodes with different voltages. Whenan electron meets a hole, it falls into a lower energy level, andreleases 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 thep�n junction. In silicon or germanium diodes, the electrons andholes recombine by a non�radiative transition which produces nooptical emission, because these are indirect band gap materials.The materials used for the LED have a direct band gap withenergies corresponding to near�infrared, visible ornear�ultraviolet light.

LED development began with infrared and red devices madewith gallium arsenide. Advances in materials science have madepossible the production of devices with ever�shorterwavelengths, producing light in a variety of colors.

LEDs are usually built on an n�type substrate, with an electrodeattached to the p�type layer deposited on its surface. P�typesubstrates, while less common, occur as well. Many commercialLEDs, especially GaN/InGaN, also use sapphire substrate.

Light extraction

The refractive index of most LED semiconductor materials isquite high, so in almost all cases the light from the LED iscoupled into a much lower�index medium. The large indexdifference makes the reflection quite substantial (per the Fresnelcoefficients). The produced light gets partially reflected backinto the semiconductor, where it may be absorbed and turnedinto additional heat; this is usually one of the dominant causes ofLED inefficiency. Often more than half of the emitted light isreflected back at the LED�package and package�air interfaces.

The reflection is most commonly reduced by using adome�shaped (half�sphere) package with the diode in the centerso that the outgoing light rays strike the surfaceperpendicularly, at which angle the reflection is minimized.Substrates that are transparent to the emitted wavelength, andbacked by a reflective layer, increase the LED efficiency. The

Some police vehicle lightbarsincorporate LEDs.

Parts of a LED

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #3refractive index of the package material should also match theindex of the semiconductor, to minimize back�reflection. Ananti�reflection coating may be added as well.

The package may be colored, but this is only for cosmeticreasons or to improve the contrast ratio; the color of thepackaging does not substantially affect the color of the lightemitted.

Other strategies for reducing the impact of the interfacereflections include designing the LED to reabsorb and reemit thereflected light (called photon recycling) and manipulating themicroscopic structure of the surface to reduce the reflectance,by introducing random roughness, creating programmed motheye surface patterns. Recently photonic crystal have also beenused to minimize back�reflections.[17] In December 2007,scientists at Glasgow University claimed to have found a way tomake LEDs more energy efficient, imprinting billions of holes intoLEDs using a process known as nanoimprint lithography.[18]

Efficiency and operational parameters

Typical indicator LEDs are designed to operate with no morethan 30–60 milliwatts [mW] of electrical power. Around 1999,Philips Lumileds introduced power LEDs capable of continuoususe at one watt [W]. These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, thesemiconductor 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 powerinput. White LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems. In 2002,Lumileds made five�watt LEDs available with a luminous efficiency of 18–22 lumens per watt [lm/W]. For comparison, aconventional 60–100 W incandescent lightbulb produces around 15 lm/W, and standard fluorescent lights produce upto 100 lm/W. (The luminous efficiency article discusses these comparisons in more detail.)

In September 2003, a new type of blue LED was demonstrated by the company Cree, Inc. to provide 24 mW at 20milliamperes [mA]. This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming thebrightest white LED commercially available at the time, and more than four times as efficient as standardincandescents. In 2006 they demonstrated a prototype with a record white LED luminous efficiency of 131 lm/W at 20mA. Also, Seoul Semiconductor has plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be approachingan order of magnitude improvement over standard incandescents and better even than standard fluorescents.[19]Nichia Corporation has developed a white light LED with luminous efficiency of 150 lm/W at a forward current of 20mA.[20]

It should be noted that high�power (≥ 1 W) LEDs are necessary for practical general lighting applications. Typicaloperating currents for these devices begin at 350 mA. The highest efficiency high�power white LED is claimed[21] byPhilips Lumileds Lighting Co. with a luminous efficiency of 115 lm/W (350 mA).

Cree issued a press release on November 19, 2008 about a laboratory prototype LED achieving 161 lumens/watt atroom temperature. The total output was 173 lumens, and the correlated color temperature was reported to be4689 K.[22]

Electrical polarity

Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity,LEDs will only light with correct electrical polarity. When the voltage across the p�njunction is in the correct direction, a significant current flows and the device is saidto be forward�biased. If the voltage is of the wrong polarity, the device is said to bereverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating currentvoltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the ACsupply.

Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltageabove this threshold. LEDs driven directly from an AC supply of more than the reverse breakdown voltage may beprotected by placing a diode (or another LED) in inverse parallel.

The manufacturer will normally advise how to determine the polarity of the LED in the product datasheet. However,these methods may also be used:

sign: + �terminal: anode (A) cathode (K)leads: long shortexterior: round flatinterior: small largewiring: red black*marking: none stripe*pin: 1 2*PCB: round square*Die placement: connector cup

(*)Less reliable methods of determining polarity

Failure modes

The most common way for LEDs (and diode lasers) to fail is the gradual lowering of light output and loss of efficiency.Sudden failures, however rare, can occur as well. Early red LEDs were notable for their short lifetime.

Nucleation and growth of dislocations is a known mechanism for degradation of the active region, wherethe radiative recombination occurs. This requires a presence of an existing defect in the crystal and isaccelerated by heat, high current density, and emitted light. Gallium arsenide and aluminium gallium arsenide

The inner workings of an LED

I�V diagram for a diode an LED will begin to emit light whenthe on�voltage is exceeded. Typical on voltages are 2�3

Volt

File:� of LED.svg

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #4are more susceptible to this mechanism than gallium arsenide phosphide and indium phosphide. Due todifferent properties of the active regions, gallium nitride and indium gallium nitride are virtually insensitive tothis kind of defect.

Electromigration caused by high current density can move atoms out of the active regions, leading toemergence of dislocations and point defects, acting as nonradiative recombination centers and producing heatinstead of light.

Ionizing radiation can lead to the creation of defects as well, which leads to issues with radiation hardening ofcircuits containing LEDs (e.g., in optoisolators)

Differentiated phosphor degeneration. The different phosphors used in white LEDs tend to degrade withheat and age, but at different rates causing changes in the produced light color, for example, purple and pinkLEDs often use an organic phosphor formulation which may degrade after just a few hours of operation causinga major shift in output color.[23]

Metal diffusion caused by high electrical currents or voltages at elevated temperatures can move metalatoms from the electrodes into the active region. Some materials, notably indium tin oxide and silver, aresubject to electromigration which causes leakage current and non radiative recombination along the chipedges. In some cases, especially with GaN/InGaN diodes, a barrier metal layer is used to hinder theelectromigration effects.

Short circuits Mechanical stresses, high currents, and corrosive environment can lead to formation ofwhiskers, causing short circuits.

Thermal runaway Nonhomogenities in the substrate, causing localized loss of thermal conductivity, can causethermal runaway where heat causes damage which causes more heat etc. Most common ones are voids causedby incomplete soldering, or by electromigration effects and Kirkendall voiding.

Current crowding, non�homogenous distribution of the current density over the junction. This may lead tocreation of localized hot spots, which poses risk of thermal runaway.

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

Thermal stress Sudden failures are most often caused by thermal stresses. When the epoxy resin packagereaches its glass transition temperature, it starts rapidly expanding, causing mechanical stresses on thesemiconductor and the bonded contact, weakening it or even tearing it off. Conversely, very low temperaturescan cause cracking of the packaging.

Electrostatic discharge (ESD) may cause immediate failure of the semiconductor junction, a permanent shiftof its parameters, or latent damage causing increased rate of degradation. LEDs and lasers grown on sapphiresubstrate are more susceptible to ESD damage.

Reverse bias Although the LED is based on a diode junction and is nominally a rectifier, the reverse�breakdownmode for some types can occur at very low voltages and essentially any excess reverse bias causes immediatedegradation, and may lead to vastly accelerated failure. 5V is a typical, "maximum reverse bias voltage" figurefor ordinary LEDS, some special types may have lower limits.

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

Color Wavelength[nm] Voltage [V] Semi�conductor Material

Infrared λ > 760 YV < 1.9 Gallium arsenide (GaAs)Aluminium gallium arsenide (AlGaAs)

Red 610 < λ < 760 1.63 < YV <2.03

Aluminium gallium arsenide (AlGaAs)Gallium arsenide phosphide (GaAsP)Aluminium gallium indium phosphide (AlGaInP)Gallium(III) phosphide (GaP)

Orange 590 < λ < 610 2.03 < YV <2.10

Gallium arsenide phosphide (GaAsP)Aluminium gallium indium phosphide (AlGaInP)Gallium(III) phosphide (GaP)

Yellow 570 < λ < 590 2.10 < YV <2.18

Gallium arsenide phosphide (GaAsP)Aluminium gallium indium phosphide (AlGaInP)Gallium(III) phosphide (GaP)

Green 500 < λ < 570 2.18 < YV < 4.0

Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)Gallium(III) phosphide (GaP)Aluminium gallium indium phosphide (AlGaInP)Aluminium gallium phosphide (AlGaP)

Blue 450 < λ < 500 2.48 < YV < 3.7

Zinc selenide (ZnSe)Indium gallium nitride (InGaN)Silicon carbide (SiC) as substrateSilicon (Si) as substrate — (under development)

Violet 400 < λ < 450 2.76 < YV < 4.0 Indium gallium nitride (InGaN)

Purple multiple types 2.48 < YV < 3.7Dual blue/red LEDs,blue with red phosphor,or white with purple plastic

Ultraviolet λ < 400 3.1 < YV < 4.4

diamond (C)Aluminium nitride (AlN)

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #5Ultraviolet λ < 400 3.1 < YV < 4.4 Aluminium gallium nitride (AlGaN)

Aluminium gallium indium nitride (AlGaInN) — (down to210 nm[24])

White Broad spectrum YV = 3.5 Blue/UV diode with yellow phosphor

Ultraviolet and blue LEDs

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

The first blue LEDs were made in 1971 by Jacques Pankove (inventor of thegallium nitride LED) at RCA Laboratories.[25] However, these devices had toolittle light output to be of much practical use. In the late 1980s, keybreakthroughs in GaN epitaxial growth and p�type doping by Isamu Akasakiand Hiroshi Amano (Nagoya, Japan)[26] ushered in the modern era ofGaN�based optoelectronic devices. Building upon this foundation, in 1993high brightness blue LEDs were demonstrated through the work of ShujiNakamura at Nichia Corporation.[27]

By the late 1990s, blue LEDs had become widely available. They have anactive region consisting of one or more InGaN quantum wells sandwichedbetween thicker layers of GaN, called cladding layers. By varying the relative InN�GaN fraction in the InGaN quantumwells, the light emission can be varied from violet to amber. AlGaN aluminium gallium nitride of varying AlN fractioncan be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yetreached the level of efficiency and technological maturity of the InGaN�GaN blue/green devices. If the active quantumwell layers are GaN, as opposed to alloyed InGaN or AlGaN, the device will emit near�ultraviolet light with wavelengthsaround 350–370 nm. Green LEDs manufactured from the InGaN�GaN system are far more efficient and brighter thangreen LEDs produced with non�nitride material systems.

With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable.Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near�UV emitters at wavelengthsaround 375–395 nm are already cheap and often encountered, for example, as black light lamp replacements forinspection 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.[28] As thephotosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about260 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 andsterilization devices.[4]

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

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

White light LEDs

There are two ways of producing high intensity white�light using LEDs. One is to use individual LEDs that emit threeprimary colors[29] – red, green, and blue, and then mix all the colors to produce white light. The other is to use aphosphor material to convert monochromatic light from a blue or UV LED to broad�spectrum white light, much in thesame way a fluorescent light bulb works.

RGB Systems

White light can be produced by mixing differentlycolored light, the most common method is to use red,green and blue (RGB). Hence the method is calledmulti�colored white LEDs (sometimes referred to as RGBLEDs). Because its mechanism is involved withsophisticated electro�optical design to control theblending and diffusion of different colors, this approachhas rarely been used to mass produce white LEDs in theindustry. Nevertheless this method is particularlyinteresting to many researchers and scientists becauseof the flexibility of mixing different colors.[30] Inprinciple, this mechanism also has higher quantumefficiency in producing white light.

There are several types of multi�colored white LEDs: di�,tri�, and tetrachromatic white LEDs. Several key factorsthat play among these different approaches includecolor stability, color rendering capability, and luminousefficiency. Often higher efficiency will mean lower colorrendering, presenting a trade off between the luminousefficiency and color rendering. For example, thedichromatic white LEDs have the best luminousefficiency (120 lm/W), but the lowest color renderingcapability. Oppositely although tetrachromatic white LEDs have excellent color rendering capability, they often havepoor 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 ofproducing light of different colors. In principle, all perceivable colors can be produced by mixing different amounts ofthree primary colors, and this makes it possible to produce precise dynamic color control as well. As more effort isdevoted to investigating this technique, multi�color LEDs should have profound influence on the fundamental methodwhich 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 decaysexponentially with increasing temperature,[31] resulting in a substantial change in color stability. Such problem is not

Blue LEDs.

Combined spectral curves for blue, yellow�green, and high brightnessred solid�state semiconductor LEDs. FWHM spectral bandwidth is

approximately 24–27 nm for all three colors.

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #6acceptable for industrial usage. Therefore, many new package designs aiming to solve this problem have beenproposed, and their results are being reproduced by researchers and scientists.

Phosphor based LEDs

This method involves coating an LED of one color(mostly blue LED made of InGaN) with phosphor ofdifferent colors to produce white light, the resultantLEDs are called phosphor based white LEDs. Afraction of the blue light undergoes the Stokes shiftbeing transformed from shorter wavelengths to longer.Depending on the color of the original LED, phosphors ofdifferent colors can be employed. If several phosphorlayers of distinct colors are applied, the emittedspectrum is broadened, effectively increasing the colorrendering index (CRI) value of a given LED.

Phosphor based LEDs have a lower efficiency thannormal LEDs due to the heat loss from the Stokes shiftand also other phosphor�related degradation issues.However, the phosphor method is still the most populartechnique for manufacturing high intensity white LEDs.The design and production of a light source or lightfixture using a monochrome emitter with phosphorconversion is simpler and cheaper than a complex RGBsystem, and the majority of high intensity white LEDspresently on the market are manufactured usingphosphor light conversion.

The greatest barrier to high efficiency is the seeminglyunavoidable Stokes energy loss. However, much effort is being spent on optimizing these devices to higher lightoutput and higher operation temperatures. The efficiency can for instance be increased by adapting better packagedesign or by using a more suitable type of phosphor. Philips Lumileds patented conformal coating process addressesfor instance the issue of varying phosphor thickness, giving the white LEDs a more homogeneous white light. Withdevelopment ongoing the efficiency of phosphor based LEDs is generally increased with every new productannouncement.

Technically the phosphor based white LEDs encapsulate InGaN blue LEDs inside of a phosphor coated epoxy. Acommon yellow phosphor material is cerium�doped yttrium aluminum garnet (Ce3+:YAG).

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiencyeuropium�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 causesphotodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturingchallenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokesshift 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, bothapproaches offer comparable brightness. Another concern is that UV light may leak from a malfunctioning light sourceand cause harm to human eyes or skin.

Another method used to produce experimental white light LEDs used no phosphors at all and was based onhomoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emitted blue light from itsactive region and yellow light from the substrate.[32]

Organic light�emitting diodes (OLEDs)

If the emitting layer material of the LED is an organic compound, it is known as an Organic Light Emitting Diode(OLED). To function as a semiconductor, the organic emitting material must have conjugated pi bonds [33] . Theemitting material can be a small organic molecule in a crystalline phase, or a polymer. Polymer materials can beflexible; 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. Somepossible future applications of OLEDs could be:

Inexpensive, flexible displaysLight sourcesWall decorationsLuminous 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[34], but their life expectancy is still far too short (<1,000hours) to be practical.

Today, OLEDs operate at substantially lower efficiency than inorganic (crystalline) LEDs[35].

Quantum Dot LEDs (experimental)

A new technique developed by Michael Bowers, a graduate student at Vanderbilt University in Nashville, involvescoating a blue LED with quantum dots that glow white in response to the blue light from the LED. This techniqueproduces a warm, yellowish�white light similar to that produced by incandescent bulbs.[36]

Quantum dots are semiconductor nanocrystals that possess unique optical properties.[37] Their emission color can betuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color onthe CIE diagram. This provides more color options and better color rendering white LEDs. Quantum dot LEDs areavailable in the same package types as traditional phosphor based LEDs.

Spectrum of a “white” LED clearly showing blue light which is directlyemitted by the GaN�based LED (peak at about 465 nm) and the morebroadband Stokes�shifted light emitted by the Ce3+:YAG phosphor

which emits at roughly 500–700 nm.

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #7

Types

The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi�color.

Miniature LEDs

These are mostly single�die LEDs used as indicators, and they come in various�sizethrough�hole and surface mount packages:

SMD2 mm3 mm (T1)5 mm (T1¾)10 mmOther sizes are also available, but less common.

Some applications (e.g. building custom LED panels for alarm clocks, dashboardsbacklight solutions and alike), require use of so�called 'unpackaged LEDs', consistingof nothing but semiconductor chip with conductive wire attached.

Common package shapes:

Round, dome topRound, flat topRectangular, 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 at4–5 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.

Five� and twelve�volt LEDs are ordinary miniature LEDs that incorporate a suitable series resistor for direct connectionto a 5 V or 12 V supply.

Flashing LEDs

Flashing LEDs are used as attention seeking indicators where it is desired to avoid the complexity of externalelectronics. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit inside whichcauses the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot.Most flashing LEDs emit light of a single color, but more sophisticated devices can flash between multiple colors andeven fade through a color sequence using RGB color mixing.

High power LEDs

See also: Solid�state lighting and LED lamp

High power LEDs (HPLED) can be driven at hundreds of mA (vs. tens of mA for otherLEDs), some with more than one ampere of current, and give out large amounts oflight. Since overheating is destructive, the HPLEDs must be highly efficient tominimize excess heat; furthermore, they are often mounted on a heat sink to allowfor heat dissipation. If the heat from a HPLED is not removed, the device will burnout in seconds.

A single HPLED can often replace an incandescent bulb in a flashlight, or be set inan array to form a powerful LED lamp.

LEDs have been developed by Seoul Semiconductor that can operate on AC powerwithout the need for a DC converter. For each half cycle part of the LED emits lightand part is dark, and this is reversed during the next half cycle. The efficiency ofHPLEDs is typically 40 lm/W.[38] As of November 2008 some HPLEDs manufacturedby Cree Inc. exceed 95 lm/W [39] (e.g. the XLamp MC�E LED chip emitting Cool

LEDs are produced in an array of shapes and sizes. The 5 mm cylindrical package (red, fifth from the left) is the most common, estimated at 80% ofworld production. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic isoften used for infrared LEDs, and most blue devices have clear housings. There are also LEDs in extremely tiny packages, such as those found on

blinkies and on cell phone keypads. (not shown).

Different sized LEDs. 8 mm, 5 mmand 3 mm, with a wooden

match�stick for scale.

High power LEDs from PhilipsLumileds Lighting Company

mounted on a 21 mm star shapedbase heatsink

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #8White light) and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent style lightsas LEDs become more cost competitive.

Multi�color LEDs

A “bi�color LED” is actually two different LEDs in one case. It consists of two diesconnected to the same two leads but in opposite directions. Current flow in onedirection produces one color, and current in the opposite direction produces theother color. Alternating the two colors with sufficient frequency causes theappearance of a blended third color. For example, a red/green LED operated inthis fashion will color blend to produce a yellow appearance.

A “tri�color LED” is also two LEDs in one case, but the two LEDs are connected toseparate leads so that the two LEDs can be controlled independently and litsimultaneously. A three�lead arrangement is typical with one common lead(anode or cathode).

RGB LEDs contain red, green and blue emitters, generally using a four�wireconnection with one common lead (anode or cathode).

The Taiwanese LED manufacturer Everlight has introduced a 3 watt RGB packagecapable of driving each die at 1 watt.

Alphanumeric LED displays

LED displays are available in seven�segment and starburst format.Seven�segment displays handle all numbers and a limited set of letters. Starburstdisplays can display all letters.

Seven�segment LED displays were in widespread use in the 1970s and 1980s,but increasing use of liquid crystal displays, with their lower power consumptionand greater display flexibility, has reduced the popularity of numeric andalphanumeric LED displays.

Considerations for usePower sources

The voltage versus current characteristics of an LED are much like any diode.Current is approximately an exponential function of voltage (see Shockley diodeequation), so a small voltage change results in a large change in current. It istherefore important that the power source gives the right voltage.

If the voltage is below the threshold or on�voltage no current will flow and theresult is a unlit LED. If the voltage is too high the current will go above themaximum rating, heating and potentially destroying the LED. As the LED heats,its voltage drop decreases (band gap decrease[40]), further increasing current.Consequently, LEDs should only be connected directly to constant�voltage sources if special care is taken. Seriesresistors are a simple way to stabilize the LED current, but wastes energy in the resistor. A constant current regulatoris commonly used for high power LEDs. Low drop�out (LDO) constant current regulators also allow the total LED stringvoltage to be a higher percentage of the power supply voltage, resulting in improved efficiency and reduced poweruse. Switched�mode power supplys are used in some LED flashlights, stabilizing light output over a wide range ofbattery voltages and increasing the useful life of the batteries.

Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2 mA, 10mA and 20 mA are common. Sub�mA indicators may be made by driving ultrabright LEDs at very low current.Efficiency tends to reduce at low currents, but indicators running on 100 cA are still practical. The cost of ultrabrightLEDs is higher than that of 2 mA indicator LEDs.

Strings of LEDs are normally operated in series LEDs, with the total LED voltage typically adding up to aroundtwo�thirds of the supply voltage, with resistor current control for each string. In disposable coin cell powered keyringtype LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not thereforebe replaced with a lower resistance type.

LEDs can be purchased with built in series resistors. These can save printed circuit board space and are especiallyuseful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistorvalue is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. AlphanumericLEDs use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, eachwith its own resistor. Seven�segment and starburst LED arrays are available in both common�anode orcommon�cathode form.

Lighting LEDs on mains

LEDs by their very nature, require direct current (DC) with low voltage, as opposed to the mains electricity from theelectrical grid which supplies a high voltage with an alternating current (AC).

A CR dropper (capacitor and resistor) followed by full�wave rectification is the usual electrical ballast with series�parallelLED clusters. A single series string minimizes dropper losses, while paralleled strings increase reliability. In practiceusually three strings or more are used.

Operation on square wave and modified sine wave (MSW) sources, such as many inverters, causes heavily�increasedresistor dissipation in CR dropper, and LED ballasts designed for sine wave use tend to burn on non�sine waveforms.The non�sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor and rectifiermakes a more suitable ballast for such use, and other options are also possible. Dedicated integrated circuits areavailable that provide optimal drive for LEDs and maximum overall efficiency.

Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greaterthan the sum of the individual LED threshold voltages. Parallel 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 whenconnecting some types of LEDs in parallel. [41]

Two color LED with single (or common)anode and dual cathode

Old calculator LED display.

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #9To increase efficiency (or to allow intensity control without the complexity of a DAC), the power may be appliedperiodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED willappear to be continuously lit.

Advantages of using LEDs

Efficiency: LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered orenergy�saving devices.[42]

Colour: LEDs can emit light of an intended colour without the use of colour filters that traditional lightingmethods require. This is more efficient and can lower initial costs.Size: LEDs can be very small (smaller than 2 mm2[43]) and are easily populated onto printed circuit boards.On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness inmicroseconds.[44] LEDs used in communications devices can have even faster response times.Cycling: LEDs are ideal for use in applications that are subject to frequent on�off cycling, unlike fluorescentlamps that burn out more quickly when cycled frequently, or HID lamps that require a long time beforerestarting.Dimming: LEDs can very easily be dimmed either by Pulse�width modulation or lowering the forward current.Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can causedamage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt burn�out of incandescentbulbs.[45]

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 longer.[46] Fluorescent tubes typically are rated at about 10,000 to15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlikefluorescent and incandescent bulbs which are fragile.Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sourcesoften require an external reflector to collect light and direct it in a usable manner.Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.

Disadvantages of using LEDs

High price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than mostconventional lighting technologies. The additional expense partially stems from the relatively low lumen outputand 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 tothreaten compact fluorescent lamps.Temperature dependence: LED performance largely depends on the ambient temperature of the operatingenvironment. Over�driving the LED in high ambient temperatures may result in overheating of the LEDpackage, eventually leading to device failure. Adequate heat�sinking is required to maintain long life. This isespecially important when considering automotive, medical, and military applications where the device mustoperate over a large range of temperatures, and is required to have a low failure rate.Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below therating. This can involve series resistors or current�regulated power supplies.[47]

Light quality: Most cool�white LEDs have spectra that differ significantly from a black body radiator like thesun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to beperceived differently under cool�white LED illumination than sunlight or incandescent sources, due tometamerism,[48] red surfaces being rendered particularly badly by typical phosphor based cool�white LEDs.However, the color rendering properties of common fluorescent lamps are often inferior to what is now availablein state�of�art white LEDs.Area light source: LEDs do not approximate a “point source” of light, but rather a lambertian distribution. SoLEDs are difficult to use in applications requiring a spherical light field. LEDs are not capable of providingdivergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of0.2 degrees or less.[49]

Blue Hazard: There is increasing concern that blue LEDs and cool�white LEDs are now capable of exceedingsafe limits of the so�called blue�light hazard as defined in eye safety specifications such as ANSI/IESNARP�27.1�05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.[50][51]

Blue pollution: Because cool�white LEDs (i.e., LEDs with high color temperature) emit much more blue lightthan conventional outdoor light sources such as high�pressure sodium lamps, the strong wavelengthdependence of Rayleigh scattering means that cool�white LEDs can cause more light pollution than other lightsources. It is therefore very important that cool�white LEDs are fully shielded when used outdoors. Compared tolow�pressure sodium lamps, which emit at 589.3 nm, the 460 nm emission spike of cool�white and blue LEDs isscattered about 2.7 times more by the Earth's atmosphere. Cool�white LEDs should not be used for outdoorlighting near astronomical observatories.

ApplicationsThe many application of LEDs are very diverse but fall into three major categories: Visual signal application where thelight goes more or less directly from the LED to the human eye, to convey a message or meaning. Illumination whereLED light is reflected from object to give visual response of these objects. Finally LEDs are also used to generate lightfor measuring and interacting with processes that do not involve the human visual system.

Indicators and signs

Status indicators on a variety of equipmentLED displays used as stadium television displays, electronic billboards and dynamic decorative displays.Traffic lights and signalsExit signsThin, lightweight message displays at airports and railway stations, and as destination displays for trains, buses,trams, and ferries.

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #10Red or yellow LEDs are used in indicator and alphanumeric displays inenvironments where night vision must be retained: aircraft cockpits,submarine and ship bridges, astronomy observatories, and in the field,e.g. night time animal watching and military field use.LEDs of all colors, including yellowish white to simulate incandescentlamps, are used for model railroading applicationsIn dot matrix arrangements for displaying messages.Because of their long life and fast switching times, LEDs have been usedfor automotive high�mounted brake lights and truck and bus brake lightsand turn signals for some time, but many high�end vehicles are nowstarting to use LEDs for their entire rear light clusters. Besides the gain inreliability, this has styling advantages because LEDs are capable offorming much thinner lights than incandescent lamps with parabolicreflectors. The significant improvement in the time taken to light up(perhaps 0.5s faster than an incandescent bulb) improves safety by givingdrivers more time to react. It has been reported that at normal highwayspeeds this equals one car length increased reaction time for the carbehind. White LED headlamps are beginning to make an appearance.

As a medium quality voltage reference in electronic circuits. The forwardvoltage drop (e.g., about 1.7 V for a normal red LED) can be used insteadof a Zener diode in low�voltage regulators. Although LED forward voltage ismuch more current�dependent than a good Zener, Zener diodes are notavailable below voltages of about 3 V.

Glowlights, as a more expensive but longerlasting and reusable alternative toglowsticks.Lumalive, a photonic textileEmergency vehicle lightingLED�based Christmas lights available indifferent colors and with low energyconsumption.LED�modules provide LEDs in a more usableform to people with less knowledge ofelectronics and soldering: the actual LEDsare contained within in protective andmountable casing, and a lead enablesconnection to power supply, typically 12volts. LED modules are available in a widerange of shapes, sizes and colors.

Lighting

Replacement light bulbsFlashlights with low energy usage and high durabilityLanternsStreet lightsLarge�scale video displaysArchitectural lightingLight source for machine vision systems, requiring bright, focused,homogeneous and possibly strobed illumination.Automotive lighting on cars, motorcyclesbicycle lightsBacklighting for LCD televisions and lightweight laptop displays. Using RGBLEDs increase the color gamut by as much as 45%.Light source for DLP projectors

Stage lights using banks of RGB LEDs to easilychange color and decrease heating fromtraditional stage lighting.Medical lighting where IR�radiation and hightemperatures are unwanted.Strobe lights or camera flashes that operate ata safe, low voltage, as opposed to the 250+volts commonly found in xenonflashlamp�based lighting. This is particularlyapplicable to cameras on mobile phones, wherespace is at a premium and bulkyvoltage�increasing circuitry is undesirable.

Invisible infrared illumination for night vision, such as many securitycameras.A ring of LEDs around a video camera, aimed forward into a retroreflectivebackground, will allow for chroma keying in video productions.

Smart lighting

Light can be used to transmit broadband data, which is already implemented inIrDA standards using infrared LEDs. Because LEDs can cycle on and off millions oftimes per second, they can, in effect, become wireless routers for data transport.[52]Lasers can also be modulated in this manner.

Non�visual applications

Grow lights using LEDs to increase photosynthesis in plants[53]

Remote controls, such as for TVs and VCRs, often use infrared LEDs.

Dropped ceiling with LED lamps

LED destination displays on buses, onewith a colored route number.

Outdoor 4 x 3 m large LED screen usedas clock.

Traffic light using LED

LED car backlights strobing at 80Hz during 1/4 s photographic

exposure time

Flashlights and lanterns that utilizewhite LEDs are becoming increasinglypopular because of their durability and

longer battery life.

LED daytime running lights of AudiA4

White LED module

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #11Movement sensors, for example in optical computer mice. The NintendoWii's sensor bar uses infrared LEDs.As light sensorsIn optical fiber and Free Space Optics communications.In pulse oximeters for measuring oxygen saturationLED phototherapy for acne using blue or red LEDs has been proven tosignificantly reduce acne over a three�month period.Some flatbed scanners use arrays of RGB LEDs rather than the typicalcold�cathode fluorescent lamp as the light source. Having independentcontrol of three illuminated colors allows the scanner to calibrate itself formore accurate color balance, and there is no need for warm�up.Furthermore, its sensors only need be monochromatic, since at any onepoint in time the page being scanned is only lit by a single color of light.As UV curing devices for some ink and coating applications.Sterilization of water and other substances using UV light.[4]

Touch sensing: Since LEDs can also be used as photodiodes, they can be used for both photo emission anddetection. This could be used in for example a touch�sensing screen that register reflected light from a finger orstylus.[54]

Opto�isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electricalisolation between two circuits. This is especially useful in medical equipment where the signals from a lowvoltage sensor circuit (usually battery powered) in contact with a living organism must be electrically isolatedfrom any possible electrical failure in a recording or monitoring device operating at potentially dangerousvoltages. An optoisolator also allows information to be transferred between circuits not sharing a commonground potential.

Light sources for machine vision systems

Machine vision systems often require bright and homogeneous illumination, so features ofinterest are easier to process. LEDs are often used to this purpose, and this field ofapplication is likely to remain one of the major application areas until price drops lowenough to make signaling and illumination applications more widespread. Barcodescanners are the most common example of machine vision, and many inexpensive onesused red LEDs instead of lasers.

LEDs constitute a nearly ideal light source for machine vision systems for several mainreasons:

Size of illuminated field is usually comparatively small and Vision systems or smartcamera are quite expensive, so cost of LEDs is usually a minor concern, comparedto signaling applications.LED elements tend to be small and can be placed with high density over flat oreven shaped substrates (PCBs etc) so that bright and homogeneous sources canbe designed which direct light from tightly controlled directions on inspectedparts.LEDs often have or can be used with small, inexpensive lenses and diffusers, helping to achieve high lightdensities and very good lighting control and homogeneity.LEDs can be easily strobed (in the microsecond range and below) and synchronized; their power also hasreached high enough levels that sufficiently high intensity can be obtained, allowing well lit images even withvery short light pulses: this is often used in order to obtain crisp and sharp “still” images of quickly�movingparts.LEDs come in several different colors and wavelengths, easily allowing to use the best color for each application,where different color may provide better visibility of features of interest. Having a precisely known spectrumallows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effect ofambient light.LEDs usually operate at comparatively low working temperatures, simplifying heat management anddissipation, therefore allowing plastic lenses, filters and diffusers to be used. Waterproof units can also easily bedesigned, allowing for use in harsh or wet environments (food, beverage, oil industries).LED sources can be shaped in several main configurations (spot lights for reflective illumination; ring lights forcoaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; domesources for diffused, omnidirectional illumination).Very compact designs are possible, allowing for small LED illuminators to be integrated within smart camerasand vision sensors.

Examples of useChristmas lights

Most LED Christmas lights (at least in 120�volt North America) are operated directly from mains electricity, with anin�line resistor (molded inside a small cylinder the same green or white color as the wire insulation) for each circuit.Older colors are operated in circuits of up to 60 LEDs, while newer or mixed colors are normally in one or two circuitsof 25, 30, or 35. An example of Halloween lights is two different sets of 70 LEDs: the orange set is divided into twocircuits with a one�kiloohm resistor each, while the purple (blue with red phosphor) set is three circuits with a 1.1 kdresistor each. Each circuit uses 2.4 watts, and from this it is derived that the LEDs are about 5 kd in total.

The alternating current can be seen in these sets by spinning one end of the string around. It is then apparent thatthe LEDs are on less than half of the time, being off when the voltage is negative (reverse�biased) or too low. Theslightly�delayed rise and slow decay of phosphors can also be seen in each flash, depending on their phosphorescence.While inexpensive, the flickering caused by this method can be annoying to some people. Additionally, theunsmoothed peak voltage of nearly 170 total volts in each cycle shortens the life of the LEDs, though they are stillrated for a service life (MTTF) of around 25,000 hours (if moisture does not rust them first). However, blue anddeep�green ones are more prone to failure, especially early in their use.

See alsoNystagmus An eye condition in which sufferers have difficulty focusing on LED displays

LED panel light source used in anexperiment on plant growth. The

findings of such experiments may beused to grow food in space on long

duration missions.

Light sources for machinevision systems.

02/05/09 http://en.wikipedia.org/wiki/Light�emitting_diode #12

Electronics portalPhotometry (optics) Main Photometry/Radiometry article—explains technical termsLED lamp—solid state lighting (SSL)Flashlight about the growing use of LED technology in FlashlightsBlinkiesThrowiesLED circuitNixie tubeLight Up the World FoundationLumalive, a photonic textileLEDs as Photodiode Light SensorsOrganic light�emitting diode

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General

Shuji Nakamura, Gerhard Fasol, Stephen J Pearton The Blue Laser Diode: The Complete Story, Springer Verlag, 2nd Edition (October 2,2000) (http://www.amazon.com/dp/3540665056/)Mills, Evan (2005). "The Specter of Fuel�Based Lighting". Science 308: 1263–1264. doi:10.1126/science.1113090. PMID 15919979.Moreno, I., "Spatial distribution of LED radiation," in The International Optical Design Conference, Proc. SPIE vol. 6342, 634216:1�7(2006).Salisbury, David F. (October 20, 2005). "Quantum dots that produce white light could be the light bulb’s successor". Exploration—TheOnline Research Journal of Vanderbilt University. http://exploration.vanderbilt.edu/news/news_quantumdot_led.htm. (More detailsregarding the use of quantum dots as a phosphor for white LEDs.)

External linksSolid State Lighting program at U.S. DOE (http://www.netl.doe.gov/ssl/faqs.htm)Photonics Sources Group, Tyndall National Institute (http://www.tyndall.ie/gan) GaN and other photonicsresearch at the Tyndall National Institute, Ireland.Solid State Lighting, Michael Shur � Rensselaer Polytechnic (http://ieee.li/pdf/viewgraphs_lighting.pdf)Applications notes about Discrete LEDs including basic driver circuits(http://www.intl�lighttech.com/applications/led�lamps)LED Circuitry Tutorial (http://www.lsdiodes.com/tutorial)LED Resistor Calculator (http://www.led�calculator.com)Light emitting diodes.org site (http://www.ecse.rpi.edu/~schubert/Light�Emitting�Diodes�dot�org/)Dendrimers in the spotlight (http://www.rsc.org/Publishing/ChemTech/Volume/2008/11/dendrimers_insight.asp)� an Instant Insight (http://www.rsc.org/Publishing/ChemTech/Instant_insights.asp) examining the use ofdendrimers in organic light�emitting diodes from the Royal Society of Chemistry

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