LIGHT EMITTING DIODE (LED)

Beside tunnel diode, varactor diode, schottky diode, step recovery diode, pin diode, there is a gun diode which is used for controlling electronic signals in the GHz range.

A P-N junction diode, which emits light when forward biased, is knows as a lights emitting diode. The emitted light may be visible or invisible. The amount of light output is directly proportional to the forward current. Thus higher the forward current, higher is the light output. The arrows, pointing away from the diode symbol represent the light, which being transmitted away from the junction.

Here and type layer is grown on P type substrate by a diffusion process. Then a thin P type layer is grown on the N-type layer. The metal connections to both the layers make anode and cathode terminals as indicated. The light energy is released at the junction, when the recombination of electrons with holes takes place. After passing through the P-region, the light is emitted through the window provided at the top of surface.

It will be inserting to know that when LED is forward bias, the electrons and holes move towards the junction and the recombination take place. After recombination, the electrons, laying in the conduction bands of N-region, fall into the holes lying in the valence band is radiated in the form of light energy. In ordinary diodes, this energy is radiated in the form of heat.

The semiconducting materials used for manufacturing light emitting diodes are gallium phosphate and gallium arsenide phosphate. The silicon and germanium is not used for manufacturing light emitting diodes because these are heat producing materials. Moreover, these materials are very poor in emitting light radiations.

The LED, radiate light in different colors such as red, green, yellow, blue, orange etc. some of the LED’s emits  infrared light also. The colors, of the emitted light, depend upon the type of the semiconductor used. Thus gallium arsenide emits infrared radiations, gallium arsenide phosphate produces either red or yellow light gallium phosphate emits red or green light and gallium nitrite produces blue light.