Fermi level (EF): definition · Chip Field Guide

Beginner

A “fill line” for electrons: energy levels below it are mostly full and levels above it are mostly empty.

Novice

The energy at which an available electron state has a 50% chance of being occupied. In pure silicon it sits near the middle of the band gap; n-type doping raises it toward $E_{\mathrm{c}}$ , p-type lowers it toward $E_{\mathrm{v}}$ . At equilibrium it is flat across a whole device.

Expert

Carrier densities follow from $E_{\mathrm{c}} - E_{\mathrm{F}}$ and $E_{\mathrm{F}} - E_{\mathrm{v}}$ . Under bias the single $E_{\mathrm{F}}$ splits into quasi-Fermi levels for electrons and holes; their separation is the applied voltage. A voltage between two terminals is a difference in their Fermi levels divided by $q$ .