The photoconductor has been called a photoresistor, photoconductive cell or, simply, a photocell. It was the first of the quantum photodetectors developed after a long period of having only thermal photodetectors. Photoconductivity was discovered by Smith in 1873 in selenium. A photoconductor involving excitation of carriers from the valence band to the conduction band is called intrinsic, and that between an impurity level and the valence band or the conduction band is called extrinsic. The first intrinsic photoconductor was demonstrated by Case using T1S in 1920. PbS, PbSe, and PbTe devices were studied in the 1930s and the 1940s. Another milestone was set by the realization of extrinsic photoconductors in Si and Ge, by Rollm and Simmons in 1952, and by Burstein el al. in 1953. The photoconductor, being a simple device, enjoys a wide spectrum of usage, from photodetector to optoelectronic switch.
The photoconductor is a light-sensitive resistor whose horizontal body, between the two contacts, is exposed to light. In this structure the active semiconductor layer, usually lightly doped, is deposited onto a semi-insulating substrate, forming an epitaxial heterostructure. In some practical devices, especially those designed as discrete components, the metal contacts have interdigitated patterns.This maximizes the exposed area with a large W, and enhances the gain with a small L. The active film can also be polycrystallme so it can simply be deposited onto an inexpensive substrate such as ceramic. Since the resistance is the main measurement, the contact resistance of the metal has to be minimized. This can be achieved by introducing a heavily doped region at the metal contact. Common semiconductors used for photoconductors are CdSe, CdS, CdTe, InSb, InP, PbS, PbSe, Ge, Si, GaAs, and GaR.