Question

How does the energy gap of an intrinsic semiconductor effectively change when doped with a (a) trivalent impurity, and (b) pentavalent impurity? Justify your answer in each case.

Hint:

Doping with trivalent impurities creates acceptor levels, while doping with pentavalent impurities creates donor levels, both affecting the energy gap.

Answer 1

(i) Doping a semiconductor with a trivalent impurity (e.g., boron) forms an acceptor level slightly above the valence band. This reduces the energy gap, facilitating electron transition from the valence band to the acceptor level and thus enhancing conductivity. (ii) Doping a semiconductor with a pentavalent impurity (e.g., phosphorus) creates a donor level just below the conduction band. This also narrows the energy gap by allowing easy excitation of electrons from the donor level to the conduction band, thereby increasing conductivity.