Light-emitting diodes (LEDs) are solid-state semiconductors that convert electrical energy directly into light in a process called electroluminescence - a process that requires much less energy than traditional light sources. When an LED is switched on, the electrical current allows electrons to recombine with holes (a convenient term for the absence of an electron where one could exist!). When an electron combines with a hole, it excites and releases photons (light). This process is achieved using a p-n junction. A p-n junction is formed by p-type and n-type semiconductor materials in very close contact. The term junction refers to the boundary interface where the two regions of the semiconductor meet. If they were constructed of two separate pieces this would introduce a grain boundary, so p-n junctions are created in a single crystal of semiconductor by doping each side to form the differing properties.

After joining p-type and n-type semiconductors, electrons near the p-n interface tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (holes) in the n region. Passing a current through the junction both provides a supply of electrons to combine with the holes (producing light) and also induces further diffusion of electrons to the p region to leave new holes.

The semiconductor material has to be thin enough to allow exit of light and is composed of either powder (bonded to the surface of a small base plate in lighting applications) or thin films as used in information displays such as TV screens. Light colour corresponds to the energy (wave-length) of the photons released and is determined by the composition and doping of the semiconductor material - a wide spectrum of colours are now available. Blends and colour switching are achieved by combining a compact array of LEDs of differing colour compositions into one unit.

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Light-emitting Diodes Introduction