In any device, a charge density that is significantly different from zero in any given region. The region containing the space charge is a space-charge region. In a semiconductor it is the region containing the depletion layers associated with the junction between two dissimilar conductivity types. In a thermionic valve the space-charge region surrounds the cathode and contains electrons not immediately attracted to the anode. These two examples of space-charge regions can exist in the devices in equilibrium under conditions of zero applied bias; they constitute potential barriers that must be overcome, when bias is applied, before the device can conduct.

Space charge also causes divergence of a beam of electrons and debunching in velocity-modulated tubes. A radial field is often used to counteract the space-charge divergence of an electron beam and results in a cylindrical beam.