In a
semiconductor or
insulator, the conduction band is the lowermost allowed
energy band that is completely unoccupied by electrons at 0K. Below the conduction band in energy lies an energy
bandgap, which is a range of energies (typically on the order of 1
eV wide for semiconductors and several eV wide for insulators) that are not
eigenenergies of the material. Below this bandgap is the
valence band, which is completely occupied 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. The number of conduction-band electrons (and valence-band holes) also depends on the concentration of dopants in the material and the intensity and wavelength of incident light.