The quasi-Fermi level, also known as the quasi-Fermi energy level, is a concept used primarily in semiconductor physics to describe the energy levels of charge carriers (electrons and holes) under non-equilibrium conditions. In a semiconductor, carriers can be in thermal equilibrium, where their energy distribution can be described by a single Fermi level.
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A quasi-Fermi level is a local electron or hole population parameter in a semiconductor driven out of equilibrium, assuming each population is internally close enough to thermal equilibrium to use a Fermi-Dirac distribution. Separate electron and hole parameters replace one common equilibrium chemical potential. Their spatial profiles depend on transport, generation and recombination as well as electrostatics. For ideal ohmic contacts with the p voltage relative to n, the contact electron electrochemical potentials obey .