In semiconductor physics, the photo–Dember effect (named after its discoverer ) is the formation of a charge dipole in the vicinity of a semiconductor surface after ultra-fast photo-generation of charge carriers. The dipole forms owing to the difference of mobilities (or diffusion constants) for holes and electrons which combined with the break of symmetry provided by the surface lead to an effective charge separation in the direction perpendicular to the surface. In an isolated sample, where the macroscopic flow of an electric current is prohibited, the fast carriers (often the electrons) are slowed down and the slow carriers (often the holes) are accelerated by an electric field, called the Dember field.