Principle of working of LED, photodiode, solar cell and zener diode.
Kindly ask different query in different thread. Also, refer the solution for LED.
LED is a heavily doped p-n junction diode which under forward bias emits spontaneous radiation. The doide is covered with a transparent cover so that the emitted light come out.
Working: When p-n junction is forward biased, the movement of majority charge carriers takes place across the junction. The electrons move from n-side to p-side through the junction and holes from p-side to n-side. therefore the concentration of minority charge carriers increased rapidly at the junction boudary, as compared to the equilibrium concentration. on recombination of hole and electrons on either side of the junction boundary, the energy is given out in the form of heat and light. The photons with energy equial to or slightly less than the band gap are emitter. the colour of the light emitted depend upon the type of materials used in making the semiconductor diode.