quasiFermi

Quasi-Fermi levels are a concept used in semiconductor physics to describe the behavior of systems that are not in thermal equilibrium. In a semiconductor at thermal equilibrium, there is a single Fermi level that defines the probability of an electron occupying a given energy state. However, when a semiconductor is excited, for example, by light or an electric current, it can become non-equilibrium. In such non-equilibrium situations, the electron and hole populations can deviate from their equilibrium distributions.

To characterize these deviations, two quasi-Fermi levels are introduced: one for electrons and one for holes. The electron quasi-Fermi level, denoted as $E_{F_n}$, describes the energy level that would correspond to the electron population if it were in thermal equilibrium, but with the actual non-equilibrium electron concentration. Similarly, the hole quasi-Fermi level, $E_{F_p}$, describes the hole population. The difference between the electron and hole quasi-Fermi levels, $E_{F_n} - E_{F_p}$, is directly related to the deviation from equilibrium and can be used to determine the electrical potential in the device.

These quasi-Fermi levels are particularly useful in analyzing phenomena such as photoconductivity, electroluminescence, and the operation of p-n junctions under bias. They provide a framework for understanding how charge carriers are distributed and transported in non-equilibrium semiconductor devices, allowing for more accurate modeling and design.