Valence and conduction bands

The valence band is the highest range of electron energies normally occupied in a solid, while the conduction band is a higher range in which electrons can move through the material and carry current.

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Band gap: The energy interval between a material’s valence-band maximum and conduction-band minimum. Its size determines how readily electrons can reach conducting states.
Energy band: A continuous range of allowed electron energies in a solid. Valence and conduction bands are two distinct energy bands.
Semiconductor: A material whose electrical conductivity lies between that of a conductor and an insulator and can be controlled. Its modest band gap enables conductivity to be tuned by doping, heat, or light.
Metal: A material with high electrical and thermal conductivity, typically arising from mobile electrons. In metals, available states near the Fermi level permit conduction without a substantial gap.
Topological insulator: A material insulating in its bulk but conducting along boundaries because of nontrivial band topology. Its boundary states depend on global properties of the occupied bands.
Fermi level: The electron chemical potential that sets occupation probabilities of available energy states. Its position relative to the bands indicates which states are occupied.
Brillouin zone: A primitive cell in reciprocal space that represents distinct crystal wavevectors. Band energies are commonly plotted across this region of wavevector space.
Intrinsic semiconductor: A pure semiconductor whose carriers arise from thermal excitation rather than intentional doping. Its carrier population comes from electrons promoted across the band gap.
Insulator: A material that strongly resists electrical current under ordinary conditions. A large gap separates occupied valence states from accessible conduction states.
Weyl semimetal: A material whose conduction and valence bands meet at isolated Weyl points in momentum space. Its crossings make band touching central rather than exceptional.