Question

Draw the "Energy bands', diagrams for a (i) pure semiconductor, (ii) insulator. How does the energy band, for a pure semiconductor get affected when this semiconductor is doped with (a) an acceptor impurity (b) donor impurity ?

Hence discuss why the 'holes, and the 'electrons' respectively, become the 'majority charge carriers'in these two cases?

Write the two process involved in the formation of p-t junction.

Answer 1

The required energy band diagram are as shown :

(a) When the semiconductor is doped with an acceptor impurity there is an additional energy level a Iittle above the top of the valence band.

(b) The donor impurity results in an additional energy level a little below the bottom of the conduction band.

(AIso accept diagrammatic representations)

In the first case, electrons, from the valence band, easily jump over to the acceptor level, leaving'holes' behind. Hence,'holes' become the majority charge carriers.

In the second case, electrons from the donor level, easily'jump over' to the conduction band. Hence, electrons become the majority charge carriers.

The two processes, involved in the formation of the p-n junction are :

(i) Diffusion

(ii) Drift