A light-emitting diode (LED) is a forwardbiased pn-junction diode formed from compound semiconductor materials such as gallium arsenide (GaAs) in which light emission can take place from direct radiative recombination of excess electron-hole pairs. A photon is emitted when an electron in the conduction band recombines with a hole in the valence band.

In infrared emitting LEDs, the encapsulating plastic lens may be impregnated or coated with phosphorus. Then, phosphorescence of the phos-phorus gives off visible light.

[Note: In an ordinary pn-junction diode, energy released in electron-hole recombination process is absorbed in the crystal structure as heat.]