Concept:When operated in reverse bias, a photodiode has a low junction capacitance and a high internal electric field that rapidly sweeps photo-generated carriers across the depletion layer, enabling switching times in the nanosecond (\( \text{ns} \)) to picosecond range.
Formula:$$t_{\text{response}} \approx t_{\text{drift}} = \frac{W}{v_{\text{sat}}} \sim 10^{-9}\text{ s} = 1\text{ ns}$$
Solution:- The reverse-bias electric field accelerates photo-generated carriers at their saturation drift velocity (\( \sim 10^7\text{ cm/s} \)).
- This produces rapid current responses to incoming light pulses, making photodiodes well-suited for high-speed optical communications and fast digital counting.
Why other options are incorrect:- Option A: All solid-state devices produce some thermal and shot noise.
- Option B: Photodiodes are passive sensors and do not provide optical amplification.
- Option C: Emitting coherent light is the function of semiconductor laser diodes, not receiving photodiodes.
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