SOLUTION

PHOTOCONDUCTIVE CELLS

• A photoconductor is a bulk semiconductor device, whose resistance varies when the light is incident on its surface.
• All semiconductors can be considered as photoconductors. This is because any semiconductor will produce electron-hole pairs (EHP) when it is irradiated by electromagnetic radiation of the optical frequencies.
• These include frequencies in the infrared, visible and ultraviolet radiations. 
• The photoconductive materials most frequently used include cadmium sulfide (CdS) and cadmium selenide (CdSc). The response time of CdS units is about 100 ms and of CdSe cells is 10 ms.

• The photoconductive cell is a  two-terminal semiconductor device whose resistance varies inversely with the intensity of light that falls upon its photosensitive material shows a typical photoconductive cell and the schematic symbol.
• These devices are also called photoresistive cells or photoresistors since the change in conductivity appears as a change in resistance.