Question

What is light emitting diode (LED)? Explain its working principle and state factors on which the colour of light emitted by it depends.

Answer 1

It is a special type $P-N$ junction diode which emits visible light of specific colour when forward biased. Hence it is called light emitting diode (LED).

Working Principle: Light emitting diode works on the phenomenon of electroluminescence. When $P-N$ junction is forward biased then electrons form the $N$-type semiconductor move towards the $P$-type semiconductor where they recombine with the holes on $P$-side. Since the electrons are in the higher conduction band on the $N$-side and holes are in the lower valence band on the $P$-side therefore during recombination, some of the energy corresponding to the difference, is given up in the form of heat and light. During recombination electrons from higher energy level conduction and fall into lower energy level (valence bond) and the energy corresponding to the forbidden energy gap $E_g$ is released. Hence for electromagnetic waves that are emitted.

$\lambda =\frac{hc}{E_g}$

where $h$ planck's constatn and $c$ is the velocity of light.

Now the number of holes in the $N$-side increases. These holes recombine with the electrons and energy is released (fig.). For silicon and germanium junctions, this emitted energy is in the form of heat and amount of energy emitted as light is insignificant. But in some specific semiconductor materials such as gallium arsenide (GaAs), gallium phosphide (GaP) and gallium arsenide phosphide (GaAsP) etc. most of the energy released during recombinations in in the form of light.

Since this semiconductor material is translucent light is emitted out and the function becomes a light source such a diode is called Light Emitting Diode (LED).

Factors: The colour of light emitted depends on the type of the material used.

For GaAs infrared radiations (invisible) are emitted.

For GaAsP, red or yellow light is emitted for Gap, red or green light is emitted.