Question

With energy band diagram ,explain the variation of fermi energy level with temperature in extrinsic semiconductor .

Answer 1

IN n-TYPE SEMICONDUCTOR.
⦁ At 0K the fermi level E_Fn lies between the conduction band and the donor level. 
⦁ As temperature increases more and more electrons shift to the conduction band leaving behind equal number of holes in the valence band. These electron hole pairs are intrinsic carriers.
⦁ With the increase in temperature the intrinsic carriers dominate the donors.
⦁ To maintain the balance of the carrier density on both sides the fermi level E_Fn gradually shifts downwards.
⦁ Finally at high temperature when the donor density is almost negligible E_Fn  is very close to E_Fi.

IN p-TYPE SEMICONDUCTOR.
⦁ At 0K the fermi level E_Fp in a p-type semiconductor lies between the acceptor level and the valence band. 
⦁ With the increase in temperature more and more holes are created in the valence band as equal number of electrons move to the conduction band.
⦁ As temperature increases the intrinsic holes dominate the acceptor holes.
⦁ Hence the number of intrinsic carriers in the conduction band and in the valence band become nearly equal at high temperature.
⦁ The fermi level E_Fp gradually shifts upwards to maintain the balance of carrier density above and below it.
⦁ At high temperature when the acceptor density become insignificant as compared to the intrinsic density, E_Fp is positioned very close to the intrinsic fermi level E_Fi but little below it.