A photoresistor is a semiconductor device that changes the value of its resistance when exposed to light. It does not have a p-n junction, therefore it has the same conductivity regardless of the direction of current flow. The phenomenon of a change in the electrical resistance of a semiconductor due to the direct action of radiation is called the photoresistive effect, or the internal photoelectric effect.
For the manufacture of photoresistors, semiconductor materials with a band gap optimal for the problem being solved are used. So, to register visible light, photoresistors made of cadmium selenide and sulfide, Se, are used. For recording infrared radiation, Ge (pure or doped with Au, Cu or Zn), Si, PbS, PbSe, PbTe, InSb, InAs, HgCdTe, often cooled to low temperatures, are used. The semiconductor is applied in the form of a thin layer on a glass or quartz substrate or cut out in the form of a thin plate from a single crystal. The semiconductor layer or plate is supplied with two electrodes and placed in a protective case.
Photoresistors are used to register weak light fluxes, when sorting and counting finished products, to control the quality and readiness of a wide variety of parts; in the printing industry to detect breaks in paper tape, control the number of sheets of paper fed into the printing machine; in medicine, agriculture and other fields.