Photo Transistor



The phototransistor has a light  sensitive collector to  base junction. A lens is     used  in  a  transistor  package to  expose  base  to  an  incident  light. When  no  light  is incident, small leakage current flows from collector to emitter called 1 due to small    thermal  generation.  This  is  very  small  current, of  the  order  of  nA.  This  is  called  a dark current.

When the base is exposed to the light, the base current is produced which is proportional to the light intensity. Such photo induced base current is denoted as h. The resulting collector current is given by,



A  phototransistor  can  be  either  a two  lead  or a  three  lead  device.  In  a  three  lead device, the base lead is brought out so that it can be used as a conventional BJT with
or without the light sensitivity feature.

In  a  two  lead  device,  the  base  is  not  electrically  available  and  the  device  use  is totally light dependent. The use of phototransistor as a two lead device is shown in
Fig.


while the Fig shows the typical collector characteristic curves.



Each  curve  on  the  characteristic  graph  is  related  to  specific  light  intensity.  The collector  current  level  increases  corresponding to  increase  in  the  light  intensity.  In most  of  the  applications  the  phototransistor  is  used  as  a  two  lead  device.  The phototransistor is not sensitive to all the light but sensitive to light within a certain    range.  The  graph  of  response  against  wavelength  is  called  spectral  response.  A typical spectral response is shown in the Fig.





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