Band diagram

"This article refers to the electronic bandgap found in the semiconductors; for discussion of the photonic band gap, see Photonic Crystal article."

In solid-state physics of semiconductors, a band diagram is a diagram showing the variation in the valence band and conduction band edges vs. some spatial dimension, often denoted "x". The Fermi level is also usually indicated in the diagram. Sometimes the "intrinsic Fermi energy", "E_i", which is the Fermi level in the absence of doping, may be shown also. The work function and electron affinity are shown on some diagrams as well. These diagrams are useful in explaining the operation of many kinds of semiconductor devices.

Metals have unfilled valence band, or greatly overlapping valence and conduction bands. Semimetals have a small overlap between valence and conduction bands, whereas semiconductors have a small band gap between them. The Fermi level is located in the middle of the band gap for an undoped semiconductor. Insulators have a large band gap, so that electrons cannot escape the valence band to enter the conduction band.