JEE Main 2020 · 5 September · morning · Single correct · +4 / −1
With increasing biasing voltage of a photodiode, the photocurrent magnitude :
- (a) Increases initially and after attaining certain value, it decreases
- (b) Increases linearly
- (c) Increases initially and saturates finally
- (d) Remains constant
Answer and solution
Answer (c) Increases initially and saturates finally
Solution
Physics sir · 4 steps
Light makes the carriers and the voltage only collects them, so the current rises and then levels off.
- Where current comes from. A photodiode works in reverse bias. Photons absorbed near the junction create electron-hole pairs. The electric field in the depletion region sweeps these carriers apart, and that flow is the photocurrent.
- Raising the bias. A larger reverse voltage widens the depletion region and strengthens the field. Fewer pairs recombine before they are collected, so at first the photocurrent grows with voltage.
- The limit. At a fixed light intensity, only a fixed number of pairs are made each second. Once the field collects nearly all of them, more voltage has nothing extra to collect, and the current stops rising.
- So. The photocurrent increases initially and then saturates. Its saturation value depends on the light intensity, not on the voltage. The answer is (c).
The idea to keep. In a photodiode the light intensity sets how many carriers exist, and the bias only decides how many get collected. Once all are collected, the current saturates.
Answer from the NTA final answer key. Solution by PrepWiser.