Light-Emitting Diodes

Light Emitting Diodes (LEDs) convert electrical power to generally visible optical power and are simply p-n-junction devices, when biased in the forward direction. They produce light through spontaneous emission whose wavelength is determined by the bandgap of the semiconductor in which the carrier recombination takes place. Unlike the semiconductor laser, generally the junction is not biased to and beyond transparency. Consequently self absorption occurs and photons are emitted in random directions. A modern LED is generally of a double-heterojunction type with the active layer being the only absorbing layer including the substrate. In addition, a plastic dome to increase the light collection cone and to focus the light is employed. Nitride-based LEDs with InGaN-actice regions span the visible spectrum from yellow to violet, as illustrated in Fig. 11.1.