Space charge is a concept in which excess
electric charge is treated as a
continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. This model typically applies when
charge carriers have been emitted from some region of a solid—the cloud of emitted carriers can form a space charge region if they are sufficiently spread out, or the charged atoms or molecules left behind in the solid can form a space charge region. Space charge usually only occurs in
dielectric media (including
vacuum) because in a conductive medium the charge tends to be rapidly neutralized or
screened. The sign of the space charge can be either negative or positive. This situation is perhaps most familiar in the area near a
metal object when it is heated to
incandescence in a
vacuum. This effect was first observed by
Thomas Edison in light bulb
filaments, where it is sometimes called the
Edison effect, but space charge is a significant phenomenon in many vacuum and
solid-state electronic devices.