Light-emitting diodes (LED) emit light in proportion to the forward current through the diode. LEDs are low voltage devices that have a longer life than incandescent lamps. They respond quickly to changes in current (10 MHz). LEDs have applications in optical-fiber communication and diode lasers. They produce a narrow spectrum of coherent red or infrared light that can be well collimated.
As an electron in the conduction band recombines with a hole in the valence band, the electron makes a transition to a lower-lying energy state and releases energy in an amount equal to the band-gap energy. Normally the energy heats the material. In an LED this energy goes into emitted infrared or visible light.