Light Emitting Diode (LED)

Structure. A light-emitting diode (LED) is a p-n junction diode, heavily doped on both sides and constructed from a semiconductor material specifically chosen (commonly gallium arsenide, gallium phosphide, or gallium arsenide phosphide, rather than plain silicon or germanium) so that its energy gap corresponds to a photon energy in, or close to, the visible part of the spectrum. It is always mounted with its junction close to the surface, inside a small, typically dome-shaped transparent (or tinted) epoxy encapsulation that both protects the junction and acts as a simple lens to help the emitted light escape.

Working: electroluminescence. An LED is operated under FORWARD bias, exactly like an ordinary rectifier diode (Section 9.10). Under forward bias, a large number of electrons are injected across the junction from the n-side into the p-side, and a large number of holes are injected from the p-side into the n-side; once inside the 'wrong' side, these injected minority carriers quickly RECOMBINE with the abundant majority carriers already present there. In an ordinary silicon or germanium diode, the energy released by each recombination event is dissipated quietly as heat inside the crystal lattice; in an LED's chosen semiconductor, that same energy is instead released predominantly as a PHOTON of light -- a process called electroluminescence -- with the photon's energy, and hence the light's colour, set by the size of the material's energy gap $E_g$ (chosen by the manufacturer through the specific compound semiconductor used). …