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Question
With what considerations in view, a photodiode is fabricated? State its working with the help of a suitable diagram.
Even though the current in the forward bias is known to be more than in the reverse bias, yet the photodiode works in reverse bias. What is the reason?
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Solution
The photodiode is special-purpose silicon diode. It is fabricated with a transparent window to expose its junction to light radiations. It always works on reverse bias condition below the breakdown voltage
When visible light of energy (hv > Eg) enters its depletion region, the electron-hole pairs are generated. These charge carriers are separated by the junction's electric field and are made to flow across the junction and cause reverse saturation current. The value of reverse saturation current depends on the intensity of incident radiation and is independent of reverse bias
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The photodiode is operated in reverse bias condition because in the reverse bias condition the change in reverse saturation current is directly proportional to the change in incident light intensity. Thus, the photodiode can be used to detect the optical signals. It cannot be done if the photodiode is forward biased.
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