LED

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LED

A LED (light-emitting diode) converts electrical energy to light by means of a semiconductor, made of a solid material, such as silicon, whose electrical conductivity when hot is as great as that of metals and very low when cold. LEDs were commonly referred to

early in their history as solid-state lamps. The light produced by LEDs is known as electroluminescence, distinguishing it from incandescence, which is characteristic of light bulbs.

Semiconductors for LEDs are made from slices of crystal so thin that their lattice, or most basic physical structure, can be easily traversed. This crystal is alloyed with materials of opposing charges, one on each side. The negative side of the semiconductor is electron rich, and the positive side is electron poor. The positive-negative junction formed by the alloyed crystal is known as a depletion layer, which is relatively inactive. The crystal is then subjected to doping, a process of destabilizing negativity and positivity of the alloys in order to affect their conductivity, so that electrons move in one direction when the semiconductor is held between two electrodes and charged with a current. The electrons shed the energy they pick up from the current in the form of photons, emitting visible light, in order to return to their normal low-energy positions. The color of the emission is determined by its spectral composition. High energy means a short wavelength, which tends toward the blue end of the spectrum. However, while a green diode peaks

at green, it can be made to emit an extended range from yellow and orange into red, depending on current flow. It appears red at low current, green at high.

The solid-state lamp

The LED is sometimes referred to as a solid-state lamp, especially in older scientific publications. Henry Round was the first to place a crystal of silicon carbide between two wires in 1907. He found that, at ten volts, it gave off a yellow light. A Russian experimenter—alternately referred to as Lossew or Lossyev—first contributed a basic understanding of the electroluminescent properties of silicon carbide, which he coupled with zinc oxide in point-contact diodes in 1923. Georges Destriau in 1936 first conceived of an AC solid-state light source, but efficiencies improved dramatically in lab research during the 1950s.

In 1990 at Cambridge University in England, physicist and engineer Sir Richard Friend made the

KEY TERMS

Dopant —A chemical impurity that is added to a pure substance in minute quantities in order to alter its properties.

Electroluminescence —A luminous discharge of high frequency brought about by photon emissions.

Rectifier —A device that converts alternating current (AC) to direct current (DC).

Semiconductor —A solid whose conductivity varies between that of a conductor (like a metal) at high temperatures and that of an insulator (such as rubber) at low temperatures.

Solid-state —Refers to the exploitation of the electric, magnetic, or light-producing capabilities of solids without depending on electron tubes.

first LED utilizing polymers (chemical compounds consisting of combinations of molecules that form crystals in repeating structural units), which are more flexible as a medium than silicon. The advantage of a polymer hinges on its chemical redundancies, artificially induced at the molecular level. Such a network of redundant material can be compared to a floating tangle of cooked noodles, but their reactions are more uniform than their appearance may suggest. When an electrical current is run through an electrically unstable polymer, its electrons all react in unison, as if a single switch flips on all the lights in an apartment building at once. Another advantage is that the wavelength of the emitted photons can easily be adjusted by altering the polymer. Some polymer chains last longer than others before being exhausted, however, and the reason for this phenomenon remains elusive.

Convenient uses and ambitious plans

LEDs show an immunity to electromagnetic interference, power surge hazards, and changes in temperature. Red LEDs are sturdy, bright enough without being too hot, and cheap enough to be conveniently used in place of more common white light sources. Green ones are used to light up phone keypads, for instance. As circuit board indicators on personal computers, they can withstand the heat stress of active integrated circuitry. LEDs also show up on electronic newscasters, are used on a large scale on building exteriors but are also available in portable forms. Laptop computers use LEDs to create the equivalent of a 12-inch monitor on a much smaller scale. Long distance phone service is renewed at switchers and loop circuits, and cable TV transmissions are aided at relay stations with a combination of lasers and LEDs known as optical coupling. LEDs provide the stimulated emission of radiation required for onset of laser action in these fiber-optics applications. AT&T/Bell Labs has produced the basic LEDs for full-color reproduction—red, green and blue (RGB)—from the same material. An array of LEDs produces LED back-lighting television sets, which were first introduced in the mid 2000s.

See also Battery; Calculator; Fiber optics; Transistor.