Abstract: A transmission photodetector operable at wavelengths greater than 0.86 micrometers comprising a substrate transparent to the radiation to be detected, at least one epitaxial intermediate layer comprising (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y As and an epitaxial p-type Ga.sub.1-y In.sub.y As detector layer. The said one intermediate layer may be p-type. If desired a second epitaxial intermediate layer comprising (Ga.sub.1-x Al.sub.x).sub.1-z In.sub.z As may be provided between the substrate and the said one intermediate layer. In the foregoing 0<x.ltoreq.1, 0<y<1, and .ltoreq.0 z< y.

Abstract: A transmission photocathode comprising a crystal substrate transparent to the radiation to be detected, at least one epitaxial crystalline intermediate layer having a lattice constant close to that of the detector layer and transparent to the radiation to be detected, and a p-type group III-V compound detector layer. Preferably the intermediate layer is p-type.