From Wikipedia, the
Blue, green and red LEDs.A light-emitting diode (LED) is a semiconductor
device that emits incoherent narrow-spectrum light when electrically biased
in the forward direction. This effect is a form of electroluminescence.
The color of the emitted light depends on the composition and condition
of the semiconducting material used, and can be infrared, visible or near-ultraviolet.
Rubin Braunstein of the Radio Corporation of America first reported on
infrared emission from gallium arsenide (GaAs) and other semiconductor
alloys in 1955. Experimenters at Texas Instruments, Bob Biard and Gary
Pittman, found in 1961 that gallium arsenide gave off infrared (invisible)
light when electric current was applied. Biard and Pittman were able to
establish the priority of their work and received the patent for the infrared
light-emitting diode. Nick Holonyak Jr. of the General Electric Company
developed the first practical visible-spectrum LED in 1962.