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The difference in energy in a substance between electron orbitals in which the electrons are not free to move (the valence band) and orbitals in which they are relatively free and will carry a current (the conduction band). In semiconductors, some electrons are sustained in the conduction band by thermal energy. Energy released when an electron in the conduction band falls into a hole in the valence band is called bandgap radiation. See also hole, semiconductor laser.