Fermi level in extrinsic semiconductors

[prudhvi mohan]

Fermi level in an extrinsic semiconductor(after attaining thermal equilibrium) is said to be invariant (constant ) , and the proof uses the fact 'no current must flow thorough any cross-section'..But I don't think its true, charge will be transported due to diffusion and this is countered by drift current( Fermi level must vary for drift to occur) ? Please tell me my mistake

 

[Sojan Francis]

diffusion happens when excess carriers are present and drift occurs when an electric field is applied that guide the carriers. If either of the situations arises, the semiconductor is no longer in thermal equilibrium and hence the concept of invariance of Fermi level is not valid.