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Photo diode























When a p-n junction is forward biased a small leakage current flows through the diode. This current occurs due to the heat generated by the bond dissociation which results in the release of holes and free electrons.

 The lifetime of these carriers before recombining is minute. However, during this intermediate time, they contribute towards the reverse biased current supply.

 If light is allowed to penetrate the depletion region of the p-n junction of a particular type (usually CdS or a Si compound), the number of electrons and holes created can be increased as opposed to the amount created during the heat generation described above.

 When the intensity of the incidence light is increased, the amount of carriers produced is also increased. Thus, the magnitude of the reverse biased current that flows through the outer circuit is also increased.

 Such diodes which are sensitive to light energy are called 'Photo diodes'.

 What happens when light penetrates a photo diode?

 The mechanism that light converts its energy to the production of new free electrons and holes can be explained as follows:
















(Photo credit : Wikimedia)



(Photo credit : olympusmicro.com)


 A photodiode should be connected to a circuit always so that it is positioned reverse biased in the circuit.

(The following part may be ignored by the beginners of electronics as it contains slightly advanced theories in electronics. However, it is always useful to keep in mind the shape of the I-V curve presented below)

 I - V Characteristics - 

 The current-voltage characteristic of a photodiode with no incident
light is similar to a rectifying diode.
 When a reverse biased current is applied, a small reverse saturation current appears. It is related to dark current as:

 ID = IS [e (qVA/kBT) - 1]
where ID is the photodiode dark current, IS is the reverse saturation
current, q is the electron charge, VA is the applied bias voltage, kB
=1.38 x 10 -23 J / K, is the Boltzmann Constant and T is the absolute temperature (273 K= 0 ºC).

 I - V curves-



































According to the above two figures three main states can be identified.

 1. V = 0. In this state current is reverse saturated.
2. V > 0 or V = + V. This is called the forward bias mode. In this region the current increases exponentially.
3. V < 0 or V = -V. This is called the reverse biased mode.
Photo diodes are used in light meters, bar code readers, counters and many other electronic appliances which measures the light intensity of a beam of light.
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