In a semiconductor or insulator, the valence band is the uppermost allowed energy band that is completely occupied by electrons at 0K. Above the valence band in energy lies an energy bandgap, which is a range of energies (typically on the order of 1eV wide for semiconductors and several eV wide for insulators) that are not eigenenergies of the material. Above this bandgap is the conduction band, which is completely unoccupied by electrons at 0K.

As temperature increases, some electrons (the precise number is described by the Fermi distribution) in the valence band have enough thermal energy to enter the conduction band, in which they are free to conduct current. These electrons leave behind unfilled eigenstates called holes in the valence band. It turns out that holes behave exactly like positively-charged mobile particles and are treated as such in solid-state physics.

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