JEE Physics 2023 Question with Solution

Given: Assertion $A$ states that photodiodes are used in forward bias for measuring light intensity. Reason $R$ states that for a $p$-$n$ junction diode, forward bias current is greater than reverse bias current, while reverse current remains very small until breakdown.

Find: The correct option for the assertion-reason pair.

1. Analyze Assertion $A$ Photodiodes are used in reverse bias, not forward bias, for measuring light intensity. In reverse bias, the photocurrent varies with the incident light intensity. Therefore, Assertion $A$ is false.

2. Analyze Reason $R$ For a $p$-$n$ junction diode, the current in forward bias is much larger than the current in reverse bias under normal operation. In reverse bias, only a small leakage current flows until breakdown voltage $V_z$ is reached. Therefore, Reason $R$ is true.

Thus, the statement combination is: $A$ is false but $R$ is true.

Therefore, the correct option is C.

Given: A photodiode is being considered for light detection.

Find: Why forward bias is not used for intensity measurement.

A photodiode is operated in reverse bias because the reverse current produced due to incident light is measurable and approximately proportional to the light intensity. If it is operated in forward bias, the normal diode current dominates and the light-dependent current cannot be measured accurately.

So, the assertion is false, while the reason remains true. Hence, the correct option is C.