Discussion
In p-n junction photodiode, the reversing biasing :
Answer: A
Reverse biasing lowers the carrier transit period through the depletion layer and reduces the depletion capacitance.
A reverse bias widens the depletion layer, and since the junction behaves like a parallel-plate capacitor, \(C = \epsilon A/W\), a larger width W gives a smaller capacitance. The reverse bias also sets up a strong electric field across the layer, which sweeps photo-generated electrons and holes across at high drift velocity, so the transit is fast. Lower capacitance and fast collection together give the photodiode a fast response.
Option B states both effects in reverse. A bias voltage does not change the doping concentrations of either side, so options C and D, which speak of changing impurity concentrations, are wrong.
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