question_answer

When a solid with a band gap has a donor level just below its empty energy band, the solid is

A)  an insulator                                      

B)  a conductor

C)  p - type semiconductor

D)  n - type semiconductor

Correct Answer: D

Solution :

                 In the extrinsic n-type semiconductor, the fifth electron of As-atom revolves around the donor atom inside the Si-crystal. As dielectric constant of silicon is very high, it is bound to the donor atom with a very small amount of energy, which is of the order of 0.045 eV. In terms of valance and conduction band, one can think that all such electrons (extrinsic carriers) create a donor energy level just below the conduction band.