PHOTODIODE
Photodiode is a reverse biased pn junction whose operation depends
on the intensity of light falling on it.
STRUCTURE
It is embedded in a clear plastic
Light is allowed to fall upon the surface across the
junction
The remaining sides are either black or embedded in metallic
case.
OPERATION
When there is no illumination, and the diode is reverse
biased a reverse saturation current flows through it.
This is due to thermally generated hole and electrons.
Reverse saturation current is in the order of µA.
Since there is no indication light this current is called as
dark current.
When light falls on the surface transfer of energy takes
place from photons to atomic structure thus generating more electron hole
pairs.
This increases the reverse current flowing through the
diode.
Further increase in light intensity increases the reverse
current.
The reverse saturation current and light intensity are in
linear relationship.
V-I CHARACTERISTICS:
This is a plot between reverse voltage and current for
different current through the diode is due to thermally generated minority
carriers.
These minority carriers are called dark current.
The current is zero when applied voltage is positive and equal
to VT.
If reverse voltage is increased in few tens a constant
current is obtained.
The spacing between the curves is equal for same light intensity.
Therefore the relationship between reverse current and light intensities is
linear.
