Light Emitting Diode(LED)
Description:
Light emitting diode is a special type of P-N junction diode that has energy gap greater than 1.8eV.
When the free electrons fall from conduction band to valence band, the energy equal to band gap (Eg) is released in the form of photons.
The photon has energy E = hv, for the photon to be visible to human eye it should have a frequency v greater than visible light hence, the energy gap should have a value greater than 1.8eV hence, Silicon and Germanium cannot be used.
Light emitting diodes are used in forward bias. This results in reduction of depletion layer allowing the free electrons to recombine with the holes i.e. minority charge carriers recombine and release energy slightly less than energy gap. This gives light of different colors.
Gallium-Arsenide LED emits infrared light.
Ga-As-P LED emits red and yellow light.
Ga-P LEDs emit red, green light.
LEDs cannot be used in reverse biased because if we reach the breakdown voltage a large amount of current will flow through it and the LED will burn out. Their reverse breakdown voltage is low about 5V.