Question

With energy band diagram , explain the variation of fermi energy level with impurity concentration in extrinsic semiconductor. 

Answer 1

Variation of Fermi level with impurity concentration

⦁ At low impurity concentration the impurity atoms do not interact with each other. Hence, the extrinsic carriers have their own discrete energy levels.
⦁ With the increase in impurity concentration the interaction of the impurity atoms start and the Fermi level varies in the following way.

⦁ IN n-TYPE SEMICONDUCTOR:- 

⦁ As the impurity atoms interacts the donor electron are shared by the neighbouring atoms.
⦁ This results in splitting of the donor level and formation of the donor band below the conduction band.
⦁ With the increase in impurity concentration the width of the band increases. At one stage it overlaps with the conduction band.
⦁ As the donor band widens the forbidden gap decreases. In the process the Fermi level shifts upwards and finally enters the conduction band as shown:-

⦁ IN p-TYPE SEMICONDUCTOR. 
⦁ With the increase in the impurity concentration the impurity atoms interact. As a result the acceptor level splits into acceptor band which gradually widens with doping level increment.
⦁ Finally the acceptor level enters the valence band. In this process the Fermi level shifts downwards and at high doping level it enters the valence band.
⦁ With the widening of the acceptor band the forbidden gap decreases as seen:-