A Photodiode is an example of an optoelectronic junction device, which implies that it is used as an electrical to optical or an optical to electrical transducer. It works on the effect of light falling onto a diode which leads to the generation of current through it. It is implemented over a special p-n junction diode by fabricating a transparent window on top of it which allows light to be incident on the diode.



A photodiode is subjected to photons in the form of light which affects the generation of electron-hole pairs. If the energy of the falling photons (hv) is greater than the energy gap (Eg) of the semiconductor material, electron-hole pairs are created near the depletion region of the diode. The electron-hole pairs created are separated from each other before recombining due to the electric field of the junction. The direction of electric field in the diode forces the electrons to move towards the n – side and consequently the holes move towards the p-side. As a result of the increase in the number of electrons on the n – side and holes on the p-side, a rise in the electromotive force is observed. Now when an external load is connected to the system, a current flow is observed through it.

The more the electromotive force created, the greater is the current flow. The magnitude of the electromotive force created depends directly upon the intensity of the incident light. This effect of proportional change in photocurrent with change in light intensity can be easily observed by applying a reverse bias.

Since photodiodes generate current flow directly depending upon the light intensity received, they can be used as photo detectors to detect optical signals. Built-in lenses and optical filters may be used to enhance the power and productivity of a photodiode.

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