Question

Explain the mechanism of a generic photodiode.

Answer 1

A photo diode is a PN junction /semiconductor device which converts light into electric current.A photo diode absorbs photons and generate current. They contain lenses which are inbuilt, optical filters etc which help for easy absorption of photons and photo diodes can be of small or larger surface area. Solar cell is an example for a photo diode having a larger surface area.

Given is the electronic symbol of a photo diode.

Photodiodes are exposed to light to reach the particular part of the device.

When a photon, which is of some amount of energy falls on the diode, and electron -hole pair is created. This is called inner photoelectric effect. The generated holes move towars anode and electrons to cathode.Thus a photocurrent is produced. The photos are allowed to fall at/near the depletion region.

The total current in a photo diode is the sum of dark crrent (under absense of light) and photo current. For improving sensitivity of the device, dark current should be minimised.

Given is the I-V characteristics of a photo diode.

Light intensity : E1<E2<E3<E4

It has 2 regions of operation.

1)Photovoltaic mode - This is zero bias mode. The circuit is ope circuired/ connected through a high impedence and thus the photocurrent flow from anode to cathode is restricted. Thus a voltage is build up in the device,anode positive with respect to cathode. When circuit is short circuited, the whole photo current flows in the crcuit. This is the photovoltaic effect and a solarcell stores energy and works this way. For maximum power output, the cell is operated in such a voltage that only small forward current< photocurrent flows.

2) Photoconductive mode - In this mode, diode is reverse biased, cathose more positive w.r.t anode. This increases width of depletion layer and electric field and thus enables the photo diode to collect electrons more quickly. This mode created more darkcurrent.Hence it exhibits more electronic noise.

Photodiodes have a wide range of applcations in CD players, Smoke detectors, remote controls etc in the day to day life.