When a semiconductor is in thermal equilibrium, the distribution function of the electrons at the energy level of E is presented by a 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 thermal equilibrium, there is no need to distinguish between conduction band quasi-Fermi level and valence band quasi-Fermi level as they are simply equal to the Fermi level.

When a disturbance from a thermal equilibrium situation occurs, the populations of the electrons in the conduction band and valence band change. If the disturbance is not too great or not changing too quickly, the bands each relax to a state of quasi thermal equilibrium. Because the relaxation time for electrons within the conduction band is much lower than across the band gap, we can consider that the electrons are in thermal equilibrium in the conduction band. This is also applicable for electrons in the valence band (often understood in terms of holes). We can define a quasi Fermi level and quasi temperature due to thermal equilibrium of electrons in conduction band, and quasi Fermi level and quasi temperature for the valence band similarly.

We can state the general Fermi function for electrons in conduction band as

• ${\displaystyle f_{0}(E,E_{\rm {F}},T)={\frac {1}{e^{(E-E_{\rm {F}})/(k_{\rm {B}}T)}+1}}}$  is the Fermi–Dirac distribution function,
• ${\displaystyle E_{\rm {Fc}}}$  is the conduction band quasi-Fermi level at location r,
• ${\displaystyle E_{\rm {Fv}}}$  is the valence band quasi-Fermi level at location r,
• ${\displaystyle T_{c}}$  is the conduction band temperature,
• ${\displaystyle T_{v}}$  is the valence band temperature,
• ${\displaystyle f_{\rm {c}}(k,r)}$  is the probability that a particular conduction-band state, with wavevector k and position r, is occupied by an electron,
• ${\displaystyle f_{\rm {v}}(k,r)}$  is the probability that a particular valence-band state, with wavevector k and position r, is occupied by an electron (i.e. not occupied by a hole).
• ${\displaystyle E}$  is the energy of the conduction- or valence-band state in question,
• ${\displaystyle k_{\rm {B}}}$  is Boltzmann's constant.

As shown in the figure below, the conduction band and valence band in a p-n junction is indicated by blue solid line in the left, and quasi Fermi level is indicated by the red dashed line.

When there is no external voltage(bias) applied to a p-n junction, the quasi Fermi levels for electron and holes overlap with one another. As bias increase, the valence band of the p-side gets pulled down, and so did the hole quasi Fermi level. As a result separation of hole and electron quasi Fermi level increased.

This simplification will help us in many areas. For example, we can use the same equation for electron and hole densities used in thermal equilibrium, but substituting the quasi-Fermi levels and temperature. That is, if we let ${\displaystyle n}$  be the spatial density of conduction band electrons and ${\displaystyle p}$  be the spatial density of holes in a material, and if the Boltzmann approximation holds, i.e. assuming the electron and hole densities are not too high, then

A current (due to the combined effects of drift and diffusion) will only appear if there is a variation in the Fermi or quasi Fermi level. The current density for electron flow can be shown to be proportional to the gradient in the electron quasi Fermi level. For if we let ${\displaystyle \mu }$  be the electron mobility, and ${\displaystyle E_{F}(\mathbf {r} )}$  be the quasi fermi energy at the spatial point ${\displaystyle \mathbf {r} }$ , then we have

• Nelson, Jenny (2003-01-01). The Physics of Solar Cells. Imperial College Press. ISBN 9781860943492.