Semiconductor Electronic: Material, Devices And Simple Circuits — Physics STD 12 Science — Question

2. The photodiode has to be reversed biased.

Explanation:
Photo diode is used to detect light.
In reverse biased condition, the width of depletion region increases as the the applied reverse bias voltage increases across the diode. So, by applying a larger voltage, more of the incident photons are converted to electric current thereby increasing the efficiency.
In forward biased condition, the width of depletion region decreases so only a small portion of the incident photons get converted to electric current and hence the efficiency is less.
Hence photo diode in reverse biased condition detects light.