Patent ID: 7365356

Claim:
A photocathode for emitting electrons in response to incident light, comprising: a semiconductor substrate of a first conductive type, said semiconductor substrate having a first surface and a second surface opposing the first surface; a first semiconductor layer of the first conductive type provided on the first surface of said semiconductor substrate; a second semiconductor layer of the first conductive type provided on said first semiconductor layer; a third semiconductor layer of a second conductive type provided on said second semiconductor layer, said third semiconductor layer having a shape such that a part in the surface of said second semiconductor layer is exposed; a surface electrode provided on said third semiconductor layer; an active layer, for reducing the work function of said second semiconductor layer, provided on the exposed part in the surface of said second semiconductor layer; and a backside electrode provided on the second surface of said semiconductor substrate, wherein the value V of the voltage applied between said surface electrode and said backside electrode divided by a minimum interval 2L between parts of said third semiconductor layer, facing each other while sandwiching the exposed part in the surface of said second semiconductor layer, is 2 (V/μm) or more.