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

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• 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:-