Quasi Fermi level

A Quasi Fermi level is a term used in quantum mechanics and especially in solid state physics to describe the change in charge carrier energy caused by the application of an external electric potential for instance in a semiconductor.

When a semiconductor is in thermal equilibrium, the distribution function of the electrons at the energy level of E is presented by fermi-Dirac distribution function. In this case the fermi level is defined as the level in which the probability of occupation of electron at that energy is 1/2.

In a non thermal equilibrium situation, the occupation of electron holes and electrons will be changed. Due to the fact that the relaxation time for electrons in sub bands of conduction band is much lower than that of the band gap, we can consider that the electrons are in thermal equilibrium in the conduction band. This is also applicable for holes in the valence band. In the case of electrons we can define a quasi fermi level due to thermal equilibrium of electrons in conduction band and quasi fermi level for holes similarly.

In thermal equilibrium electron quasi fermi level and hole quasi fermi level and fermi level are equal.