Electronic Devices - Special-Purpose Diodes - Discussion

Si and Ge are indirect band gap semiconductor, GaAs is direct band gap material, indirect band gap material does not radiate light, but direct band gap radiates. So GaAs is used.

Band = orbit . (another name for orbit is band.)
Valency band = last orbit in an atom.

When voltage is applied to the LED the electrons absorbs this energy and jumps from valence band(orbit) to conduction band.

With in very nano seconds these electrons again jumps from conduction band to valency band and releases the absorbed energy in form of heat or light.

In case of GE or silicon the energy emitted will be in the form of heat.
In case of gallium the energy emitted is in the form of light.

So gallium and one or more of the following elements are used to make LED.

GaAs --> infrared.
AlGaAs --------> red
GaP -----------> GREEN
AlGaInP --------> YELLOW
AlGaInN --------> BLUE

Electrons in the conduction band jump to valence band, thus emit energy which is either in the form of heat or light. In case LED is made up of Silicon OR Germanium the energy emitted will be in the form of heat. In case of gallium the energy emitted is in the form of light so GaAs, Gallium phosphide is used to make LED.

Greater energy being released due to its wider energy gap.