A space charge region in a semiconductor due to the fixed ionized dopants, resulting from insufficient local charge carriers. Depletion layers are inevitably formed at the interface between two dissimilar conductivity types of semiconductor, in the absence of an applied voltage (see p-n junction), and at a metal-semiconductor contact. The width of the depletion layers increases when reverse bias is applied; the depletion-layer capacitance is the capacitance associated with a given depletion layer, which effectively acts as a dielectric when depleted of mobile carriers. Reverse-biased p-n junctions or Schottky diodes can therefore be used as voltage capacitors (see varactor). A depletion layer can also form at the surface of a semiconductor of given conductivity type under the influence of an electric field (see MOS capacitor).