Question

Why is that a non equilibrium has two Fermi energies? And in equilibrium has only one Fermi energy?

Answer 1

Okay, I will try to explain this in simple words. In non equilibrium scenario, the fermi level is close to conduction band than being close to middle of band gap (which is the case in equilibrium level). Here the non equilibrium means that the concenteration of holes and electrons is not balanced. Now, In general ,it is stated that fermi energy corresponds to that level which has a probability of 1/2 being occupied (this is what fermi energy states in general). Therefore, for semiconductors fermi energy must be located between valence energy band and conduction band.

Under these conditions, the concentrations of electron and holes are no longer montiored and one can be more than other. Therefore, we need to have fermi energy levels - one for electrons and one for holes. I think you have an idea now why there are two fermi energies in non equilibrium state. There are some equations involved also but to have a basic idea is enough.

-------------------------------------------------------------------------------------------------------------------------------

In equilibrium scenarios, the concenteration of electrons and holes is well monitored and is kept in equilibrium. This is the reason, we just need single fermi energy level.