What is a Photo Diode

A photo diode is a p-n junction semi conductor device whose primary function is the detection of light. This diode is used in many counting and switching applications along with being employed in optical receivers and demodulators.

A photo diode is always used in reverse bias conditions. A P-N junction employed under reverse bias only allows a small current to flow through it. This is known as the reverse leakage current and is due to the action of the minority carriers. This current is independent of the bias applied and remains constant up to the point of breakdown.

It is very simple to understand the operation of photo diode. When light falls on the junction, it increases the number of minority carriers in the device by providing sufficient energy to knock the electrons out of the valence shells. This increase in the population of minority carriers causes an increase in the magnitude of the reverse leakage current. Thus optical energy is converted in to electrical energy. This electrical energy can be recorded as voltage drop fluctuations by using a series resistor in the outer circuit and taking voltage readings across it.

In conditions of luminance, a reverse current continuous to flow through the circuit with its magnitude depending upon the intensity of the incident light, even if the reverse bias is reduced to zero. This property of the photo diode makes it a photo conductive device and allows it to be used in the detection of light. This is because as soon as the incident light is cut off, the reverse current will go down to zero as there is no reverse bias. 

A photo diode is either made semi transparent or the junction is mounted under an optical lens to improve its sensitivity towards light. It has a very fast response and thus is very useful in high speed counter applications in industry. A path of light can be established that is incident on the photo diode and the objects to be counted are made to pass through the light. Each time an object crosses the path of light, the light incident on the diode gets cut off for a very small fraction of a second. Since a photo diode is capable of responding to such a small change, a fluctuation in current will result in the outer circuit of the diode. This fluctuation can be easily recorded by a digital counter.

The current in a photo diode increases linearly with luminance. This linearity makes it very useful for use in optical receivers and demodulators in both terrestrial and deep space optical communication systems.

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