The theory of an internal photoemission effect (IPE) in Schottky junctions has been reviewed, and the existing Vickers model has been commented and enriched. Indeed, since the modified Fowler equation is very often used to describe the IPE even when approximations on which it is based are not met, we have derived precise conditions that allow the use of the modified Fowler equation and, possibly, as it may change in the case where these conditions were not verified. Our theory shows how the performance of the surface-illuminated Schottky devices can be optimized, and we propose an analytical formulation able to calculate the metal thickness maximizing the efficiency of the devices. In addition, we prove how the minimum detectable optical power is closely linked to the area of the metal-semiconductor junction. The model has been applied to describe the theoretical limitations on the performance of surface …