PHOTO CONDUCTIVE CELL

 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