PHOTO DIODES BY JAYASRI K. (221710303019)

Photo diodes come under the category of photo junction diodes i.e. the type of photo devices which are generally semiconductor devices using light to control the flow of electrons or holes across junctions. They are basically P-N junction light sensors which are sensitive to visible and infrared light. Thus, when light is incident on a photodiode, the electrons and holes are separated allowing the junction to conduct. Its symbol is as below.

Operating Principle:

A photo diode is reverse biased so that a very low leakage current flows i.e. it is biased against its easy flow of direction of current. If a photon of sufficient energy is made incident on the diode at its junction, an electron is freed. And, if it possesses enough energy, it may even pass over the energy barrier causing a small leakage current to flow. 

The amount of current is proportional to the amount of illumination of the junction. 

A photo diode may be operated and biased in two modes: photo voltaic mode and photo-conductive mode. 

Photo Voltaic Mode: When photons are incident, a voltage is produced and is amplified by the operational amplifier. Apart from the current generated from heat, there's no basic leakage current since there's no DC bias across the diode. 

This configuration gives a high impedance load to the photo diode which thus results in a wider range of current relative to the intensity of the light which was incident. 

Photo Conductive Mode: In this mode, the photo diode is DC biased , and the current flowing through the diode is due to the DC bias. And,  the light sensing is converted to voltage by the resistor and is amplified by the operations amplifier. 

This configuration widens the depletion region since the applied bias decreases the capacitance of the photo diode. 

Characteristics:

When there is no light, the I-V characteristic of a photo diode is similar to that of a normal diode. There is an exponential increase in current when a photo diode is forward biased. When reverse biased, a small leakage current called reverse saturation current will appear and cause an increase the depletion region.