A light-emitting-diode (LED),is a semiconductor diode that emits light when an electric current is applied in the forward direction of the device, as in the simple LED circuit. The effect is a form of electroluminescence where incoherent and narrow-spectrum light is emitted from the p-n junction in a solid state material.
LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. 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 radiation pattern and assist in reflection.The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. Besides lighting, interesting applications include using UV-LEDs for sterilization of water and disinfection of devices,and as a grow light to enhance photosynthesis in plants.
History
Discoveries and early devices

The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round of Marconi Labs when he noticed electroluminescence produced from a crystal of silicon carbide while using a cat's-whisker detector.[6] 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,[7][8] 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) and other semiconductor alloys in 1955.[9] Braunstein observed infrared emission generated by simple diode structures using GaSb, GaAs, InP, and Ge-Si alloys at room temperature and at 77 kelvin.

In 1961, experimenters Bob Biard and Gary Pittman working at Texas Instruments,[10] found 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 diode.

The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General Electric Company. He later moved to the University of Illinois at Urbana-Champaign.[11] Holonyak is seen as the "father of the light-emitting diode".[12] M. George Craford, a former graduate student of Holonyak's, invented the first yellow LED and 10x brighter red and red-orange LEDs in 1972.[13] Up to 1968 visible and infrared LEDs were extremely costly, on the order of US $200 per unit, and so had little practical application. [14] The Monsanto Corporation was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide in 1968 to produce red LEDs suitable for indicators. [15] Hewlett Packard (HP) introduced light-emitting diodes (LED) in 1968, initially using GaAsP material supplied by Monsanto. The technology proved to have major applications for alphanumeric displays and was integrated into HP’s early handheld calculators.

[edit] Practical use
Some police vehicle lightbars incorporate LEDs.

The first commercial LEDs were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory 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 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. The invention and development of the high power white light LED led to use for illumination (see list of illumination applications).

Most LEDs were made in the very common 5 mm T1³⁄₄ and 3 mm T1 packages, but with increasing power output, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs.

[edit] Continuing development

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

The development of LED technology has caused their efficiency and light output to increase exponentially, with a doubling occurring about every 36 months since the 1960s, in a similar way to Moore's law. 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 Law after Dr. Roland Haitz.

[edit] LED technology
Parts of an LED
The inner workings of an LED
I-V diagram for a diode an LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 2-3 Volt

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

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for 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 arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

[edit] Light extraction

The refractive index of most LED semiconductor materials is quite high, so in almost all cases the light from the LED is coupled into a much lower-index medium. The large index difference makes the reflection quite substantial (per the Fresnel coefficients). The produced light gets partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat; 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. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The refractive index of the package material should also match the index of the semiconductor, to minimize back-reflection. An anti-reflection coating may be added as well.

The package 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 crystal have also been used to minimize back-reflections.[17] In December 2007, scientists at Glasgow University claimed to have found a way to make LEDs more energy efficient, imprinting billions of holes into LEDs using a process known as nanoimprint lithography.[18]

[edit] Efficiency and operational parameters

Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts [mW] of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt [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, Lumileds made five-watt LEDs available with a luminous efficiency of 18–22 lumens 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 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 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 efficiency of 131 lm/W at 20 mA. Also, Seoul Semiconductor 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.[19] Nichia Corporation has developed a white light LED with luminous efficiency of 150 lm/W at a forward current of 20 mA.[20]

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[21] by Philips Lumileds Lighting Co. with a luminous efficiency of 115 lm/W (350 mA).

Cree issued a press release on November 19th, 2008 about a laboratory prototype LED achieving 161 lumens/watt at 350 mA (Over 10 times more efficient than incandescent lightbulbs). Output was 173 lumens. Power works out to 1.075 watts. Voltage drop works out to 3.07 volts. Correlated color temperature was reported to be 4689 K.[22]

[edit] 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-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 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.

Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage above this threshold. 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 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     short
exterior:     round     flat
interior:     small     large
wiring:     red     black
*marking:     none     stripe
*pin:     1     2
*PCB:     round     square
*Die placement:     connector     cup

(*)Less reliable methods of determining polarity

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.

[edit] 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, where the radiative recombination occurs. This requires a presence of an existing defect in the crystal and is accelerated by heat, high current density, and emitted light. Gallium arsenide and aluminium gallium arsenide are more susceptible to this mechanism than gallium arsenide phosphide and indium phosphide. Due to different properties of the active regions, gallium nitride and indium gallium nitride are virtually insensitive to this kind of defect.

    * Electromigration caused by high current density can move 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 radiation can lead to the creation of defects as well, which leads to issues with radiation hardening of circuits containing LEDs (e.g., in optoisolators)

    * Differentiated phosphor degeneration. The different phosphors used in white LEDs tend to degrade with heat and age, but at different rates causing changes in the produced light color, for example, purple and 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.[23]

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

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

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

    * Current crowding, non-homogenous distribution of the current density over the junction. This may lead to creation 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, causing partial 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 package reaches its glass transition temperature, it starts rapidly expanding, causing mechanical stresses on the semiconductor and the bonded contact, weakening it or even tearing it off. Conversely, very low temperatures can cause cracking of the packaging.

    * Electrostatic discharge (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 sapphire substrate are more susceptible to ESD damage.

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