Abstract: A semiconductor switching device, which makes use of a hot electron emitter to give high speed operation, comprising a body (3) of intrinsic semiconductor material carried on a substrate (1) of n-type conductivity and a hot electron emitter (11, 13) which, when a bias potential is applied between the emitter and substrate, injects hot electrons into the body with a sufficient energy to generate electron-hole pairs in the body by collision of injected electrons with valence electrons. The device further includes a p-type conductivity region (7) contiguous with the body (3) to which, when appropriately biassed with respect to the substrate, the generated holes diffuse, the generated electrons diffusing to the substrate (1).