Light-emitting diode

A light-emitting diode or LED is a semiconductor device that emits incoherent monochromatic light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color depends on the semiconducting material used, and can be near-ultraviolet, visible or infrared. Nick Holonyak Jr. (1928 - ) developed the first practical visible-spectrum LED in 1962.[1] (http://www.si.edu/lemelson/Quartz/inventors/holonyak)

LED Technology

A LED is a special type of semiconductor diode. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a pn junction. Charge-carriers (electrons and holes) are created by an electric current passing through the junction, and release energy in the form of photons as they recombine. The wavelength of the light, and therefore its colour, depends on the bandgap energy of the materials forming the pn junction. A normal diode, typically made of silicon or germanium, emits invisible far-infrared light, but the materials used for a LED have bandgap energies corresponding to near-infrared, visible or near-ultraviolet light.

Unlike incandescent bulbs, which can operate with either AC or DC, LEDs require a DC supply of the correct polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward biased. The voltage across the LED in this case is fixed for a given LED and is proportional to the energy of the emitted photons. 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.

Conventional LEDs are made of inorganic minerals such as:

• aluminium gallium arsenide (AlGaAs) - red and infrared
• gallium arsenide/phosphide (GaAsP) - red, orange and yellow
• gallium nitride (GaN) - green
• gallium phosphide (GaP) - green
• zinc selenide (ZnSe) - blue
• indium gallium nitride (InGaN) - blue
• silicon carbide (SiC) - blue
• diamond (C) - ultraviolet
• silicon (Si) - under development

LED development began with infrared and red devices, and technological advances have made possible the production of devices with ever shorter wavelengths.

Blue LEDs became available in the late 1990s. They can be added to existing red and green LEDs to produce white light. Zinc selenide (ZnSe) LEDs can produce white light by emitting blue light from the pn junction which is then mixed with red-to-green light created by photoluminescence in the ZnSe.

The most recent innovation in LED technology is a device that can emit ultraviolet light. When ultraviolet light illuminates certain materials, these materials will fluoresce or give off visible light. White light LEDs have been produced by building ultraviolet elements inside material that fluoresces to produce white light.

Ultraviolet and blue LEDs are relatively expensive compared to the more common reds, greens, yellows and infrareds and are thus less commonly used in commercial applications.

The semiconducting chip is encased in a solid plastic lens, which is much tougher than the glass envelope of a traditional light bulb or tube. The plastic may be coloured, but this is only for cosmetic reasons and does not affect the colour of the light emitted.

In the last few years (up to 2003) there has been much research into organic LEDs or OLEDs, which are made of semiconducting organic polymers. The best efficiency of an OLED so far is about 10%. These promise to be much cheaper to fabricate than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a colour graphic display.

LED Applications

• thin, lightweight message displays (e.g. in public information signs)
• status indicators (e.g. on/off lights)
• traffic signals
• Bicycle lighting.
• illumination (e.g. flashlights [a.k.a. torches, U.K.], backlights for LCD displays)
• signaling/emergency beacons and strobes
• infrared remote controllers
• sensors (e.g. mechanical and optical mice)
• LED printers

LEDs offer benefits in terms of maintenance and safety. The typical working lifetime of a device, including the bulb, is ten years, which is much longer than the lifetimes of most other light sources. LEDs give off less heat than incandescent light bulbs and are less fragile than fluorescent lamps. Since an individual device is smaller than a centimeter in length, LED-based light sources used for illumination and outdoor signals are built using clusters of tens of devices.

Incandescent light bulbs for traffic signals and pedestrian crosswalks are gradually being replaced by LED clusters.

Lighting systems using incandescent bulbs are cheap to buy but inefficient, generating from about 16 lumen per watt for a domestic tungsten bulb to 22 lm/W for a halogen bulb. Fluorescent tubes are more efficient, from 50 to 100 lm/W for domestic tubes, allowing large energy savings, but are bulky and fragile and require starter circuits. LEDs are robust and moderately efficient, up to 32 lumen per watt, but are expensive, although their cost is falling. The technologies for LED production are developing rapidly.

The largest LED display in the world is 36 m high, at Times Square, Manhattan.