The photo conductive cell is a semiconductor device whose resistance varies inversely with the intensity of light that falls upon its photosensitive material. Fig. shows typical photoconductive cell and the schematic symbol. These devices are also called photoresistive cells or photoresistors, since the change in conductivity appears
as a char in resistance.
The device is constructed by vapour depositing or sintering the
photoconductive material onto a ceramic substrate. The photoconductive material
used is usually cadmium compounds such as cadmium suiphide (CdS) or cadmium selenide (CdSe). This photoconductive material with substrate is enclosed with suitable enclosure taking photoconductive material contacts as metallic leads. The glass window or lens is added at the top of the enclosure to pass light on photoconductive material.
As we know that in semiconductors, energy gap exists between conduction electrons and valence electrons. When photons are absorbed in a photoconductive material, due to impartion of the photo energy, electrons are excited into conduction band, reducing the resistance of the photoconductive material, as shown in the Fig