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In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature, while the conduction band is the lowest range of vacant electronic states.
The name "valence band" was coined by analogy to chemistry, since in semiconductors (and insulators) the valence band is built out of the valence orbitals. In a metal or semimetal, the Fermi level is inside of one or more allowed bands. In semimetals the bands are usually referred to as "conduction band" or "valence band" depending on whether ...
The band gap (usually given the symbol ) gives the energy difference between the lower edge of the conduction band and the upper edge of the valence band. Each semiconductor has different electron affinity and band gap values. For semiconductor alloys it may be necessary to use Vegard's law to calculate these values.
The band gap is called "direct" if the crystal momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice.
At the junction of two different semiconductors there is a sharp shift in band energies from one material to the other; the band alignment at the junction (e.g., the difference in conduction band energies) is fixed. At the junction of a semiconductor and metal, the bands of the semiconductor are pinned to the metal's Fermi level.
Based on the energy eigenvalues, conduction band are the high energy states (E>0) while valence bands are the low energy states (E<0). In some materials, for example, in graphene and zigzag graphene quantum dot, there exists the energy states having energy eigenvalues exactly equal to zero (E=0) besides the conduction and valence bands. These ...
The band offsets are determined by two kinds of factors for the interface, the band discontinuities and the built-in potential. These discontinuities are caused by the difference in band gaps of the semiconductors and are distributed between two band discontinuities, the valence-band discontinuity, and the conduction-band discontinuity.
Band diagram of semiconductor-vacuum interface showing electron affinity E EA, defined as the difference between near-surface vacuum energy E vac, and near-surface conduction band edge E C. Also shown: Fermi level E F, valence band edge E V, work function W.