A photodiode is operated in the reverse bias. Why?

a) Photocurrent flows only in the reverse direction

b) Thermally generated current is large in reverse bias as compared to the photocurrent

c) Photocurrent is large in the forward bias as compared to the reverse bias current

d) Photocurrent is significant in reverse bias as compared to the forward bias current

Answer: d) Photocurrent is significant in reverse bias as compared to the forward bias current

Solution:

If a photodiode is connected in the forward bias direction, it will conduct pretty much like a normal diode. When a photodiode is reverse biased, the width of the depletion and a small reverse current (dark current) flows through the diode. Now, if a light is incident on the junction, electron-hole pairs are generated in the depletion layer in a big amount (due to broad depletion layer) and these charge carriers can easily cross the barrier and it will contribute to the current across the diode. We can say that in reverse bias, the diode changes the incident light to current, mainly due to the broad depletion layer i.e. photocurrent is significant in reverse bias as compared to the forward bias current.

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