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photodiode

Introduction :


A photo diode is a semi-conductor device, with a p-n junction and an intrinsic layer between p and n layers. It can be used as a light detector, which involves the conversion of light into current or voltage depending on mode of operation.


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construction :


3

 

Working :


When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. If it occurs at the junction these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photo current is produced. The total current through the photo diode is the sum of the dark current (current that is generated in the absence of light) and the photo current, so the dark current must be minimized to maximize the sensitivity of the device.



Mainly it is operated in two modes


1. Photo voltaic mode :


It is also called as zero bias mode i.e.  no external bias is applied to the diode, the flow of current out of the device is restricted and builds voltage. a traditional solar cell is just a large area photo diode. in this mode dark current is at minimum.



2. Photo conductive mode (reverse bias) :untitled-2


 

The application of reverse voltage increases the width of the depletion layer. photo diodes exhibit their fastest switching speeds when operated in photo conductive mode.




[caption id="attachment_902" align="alignnone" width="431"]9ffa076c5d8f886c319e944239368ca0_14_01_photonics_and_lasers-4 characteristics of photo diode[/caption]

other than these two modes another type of photo diode is also available  that is  AVALANCHE DIODE 



 Avalanche diode operation :


It a highly sensitive semiconductor electronic device operated in high reverse voltage.


Advantages over above two modes :




  • High Sensitivity and Low noise

  • High-Speed response

  • Low-Light Level Measurement


avalancheLight enters the un-doped region of the avalanche photo diode and causes the generation of hole-electron pairs. Under the action of the electric field the electrons migrate towards the avalanche region. Here the electric field causes their velocity to increase to the extent that collisions with the crystal lattice create further hole electron pairs. In turn these electrons may collide with the crystal lattice to create even more hole electron pairs. In this way a single electron created by light in the un-doped region may result in many more being created.

The avalanche photo diode has a number of differences when compared to the ordinary p-i-n diode. The avalanche process means that a single electron produced by light in the  un-doped region is multiplied several times by the avalanche process. As a result the avalanche photo diode is far more sensitive.

characteristics of photo diode :


avalanche-char

 
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