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Thursday, 12 July 2012

Photo Diode

Photo Diode
Photo diode is a PN junction device that works in the ‘reversed biased’ condition when light falls on it
These are small in size and durable
Their life time is very long
These are more efficient
Circuit diagram:
A photo diode is construct just like a semiconductor conventional PN junction diode. Except one thing, that a transparent Lenz is attached over its PN junction to focus light rays on it. Light passes through this transparent Lenz and falls on PN junction.

Operation of photo diode
Photo diode normally operates in reverse bias mode.
We know that when a junction is reverse biased than ‘reverse saturation current’ passes through the junction, that flows due to pairs of holes & electrons produced due to heat.
When small amount of is light is falling on the junction, then it resistance is very high, at this condition, a minor current passes through junction that is called ‘dark current’
But when the intensity of light increase, the minority carriers obtain thermal energy.
As a result, the resistance of junction is decreased and high quantity of covalent bond breaks, it means more pairs of holes and electrons are produced and reverse current is increase.
As the light intensity increases in same speed
Materials used for photo diode
(Photo diode _ conductive materials)
Cadium sulphide
Cadium selenoide
Cadium taloride
Lead sulphide
Thallium sulphide
V/I Characteristics curves:

Optional communication devices
CD players
Punched card readers
Movies devices
Logic circuit



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