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  4. Assertion: A p-n junction with reverse bias can be used as a photodiode to measure light intensity Reason: In a reverse bias condition the current is small but is more sensitive to changes in incident light intensity.

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Q. Assertion : A $p-n$ junction with reverse bias can be used as a photodiode to measure light intensity
Reason : In a reverse bias condition the current is small but is more sensitive to changes in incident light intensity.

AIIMSAIIMS 2006Semiconductor Electronics: Materials Devices and Simple Circuits

Solution:

Photo-diode is a reverse biased $p-n$ junction diode as shown in figure. At the $p-n$ junction there exists a junction field which at
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equilibrium, does not permit the flow of charge carriers across the junction. When such a $p$-n diode is illuminated with light photons having energy $h v>E_{g}$ and intensities $I _{1}, I _{2}, I _{3}$, etc., the electron and hole pairs generated in the depletion layer (or near the junction) will be separated by the junction field and made to flow across the junction. There would be a change in the reverse saturation current as shown in figure. Hence, a measurement of the change in the reverse saturation current on illumination can give the values of the light intensity.
image