Photo-Diode



 The  photodiode  is   a  semiconductor  p-n  junction  device  whose  region  of operation  is  limited  to  the  reverse  biased  region.  The  Fig  shows  the  symbol  of photodiode while the Fig shows the working principle of photodiode.

The   photodiode  is   connected  in  reverse   biased   condition.   The  depletion region width is large. Under normal condition, it carries small reverse current due to
minority  charge  carriers.  When  light  is  incident  through  glass  window  on  the  p-n junction, photons in the light bombard the p-n junction and some energy is imparted
to  the  valence  electrons.  Due  to  this,  valence  electrons  are  dislodged  from  the covalent   bonds   and   become   free   electrons.  Thus   more   electron-hole   pairs   are generated.  Thus  total  number  of  minority  charge  carriers  increases  and  hence  the reverse current increases. This is the basic principle of operation of photodiode.




The  reverse  current  without  light  in  diode  is  in  the  range  of  micro  amperes.  The change  in this current  due to the light  is  also in the  range of micro amperes. Thus     such a change can be significance observed in the reverse current. If the photodiode
is  forward  biased,  the  current  flow through  it  is  in  mA.  The  applied  forward  biased voltage takes the control of the current instead of the light. The change in forward
current due to light is negligible and can not be noticed. The resistance of forward
biased diode is not affected by the light. Hence to have significant effect of light on
the  current  and  to  operate  photodiode  as  a variable  resistance  device,  it  is  always connected in reverse biased condition.




The Fig shows the small signal model for photodiode. Photodiode is represented by an ideal junction diode in parallel with a current source which is proportional to the light intensity.


Advantages


The advantages of photodiode are,

1.Can be used as variable resistance device.

2.Highly sensitive to the light.


3.The speed of operation is very high. The switching of current and hence the resistance value from high to low or otherwise is very fast.

Disadvantages

The various disadvantages of photodiode are,

1.The dark current is temperature dependent.

2.The overall photodiode characteristics are temperature dependent hence have poor

3.temperature stability.

4.The  current  and  change  in  current  is  in  the  range  of  1  which  may  not  be
sufficient to drive other circuits. Hence amplification is necessary.

Photodiode Applications

The two commonly used systems using photodiode are alarm system and a counting system.

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