Bandgap reduction mechanism in heavily doped semiconductors

Both the bandtailing and rigid band models of bandgap reduction are reviewed with particular emphasis on the combined density of states (conduction and impurity band) which are to be expected. The rigid band model based on the stored electrostatic energy of interaction between free electrons and the donor ions gives quantitative predictions in agreement with the results from a wide variety of experiments on majority and minority carriers. It appears to be the major mechanism for bandgap reduction.