Light Emitting Diode (LED)

An LED is a p-n junction diode that emits visible or invisible light when it is FORWARD biased -- since electrical energy is converted directly into light energy, this process is called electroluminescence. When forward biased, conduction-band electrons from the n-side and valence-band holes from the p-side diffuse across the junction; once across, they become EXCESS minority carriers in the region they entered (electrons in the p-side, holes in the n-side) and recombine with the majority carriers already there. Recombination releases energy either as heat (non-radiative recombination) or, in materials chosen for this purpose, as a photon of energy $h\nu$ (radiative recombination) -- it is this radiative pathway that an LED is built to maximise. The colour of the emitted light is set directly by the material's band gap energy $E_g$, since the photon energy released is (essentially) $E_g$ itself: this is why LEDs come in a range of colours by choice of material -- blue from SiC, green from AlGaP, red from GaAsP, and (more recently) white from GaInN. LEDs have wide use as indicator lamps, seven-segment displays, traffic and emergency-vehicle signals, and remote controls. …