Question

Why is a photo-diode invariably reverse biased, when it is used as a photo-detector?

• A. The power consumption is much reduced compared to reverse biased condition
• B. Electron hole pairs can be produced by the incident photons only if the photo diode is reverse biased
• C. Light variations can be converted into current variations only if the photo diode is reverse biased
• D. When photons are incident on the diode, the fractional change in the reverse current is much greater than the fractional change in the forward current
• E. The photo diode will be spoilt if it is operated under forward biased condition

Answer 1

The correct option is D When photons are incident on the diode, the fractional change in the reverse current is much greater than the fractional change in the forward current

Whether the photo-diode is reverse biased or forward biased, the number of electron hole pairs produced by the incident photons is the same. In other words, the change in the diode current is the same in both cases. But in the reverse biased condition the current drawn by the diode in the absence of the photons is extremely small, of the order of nanoamperes or microamperes whereas in the forward biased condition this current is significant, of the order of tens of milliamperes. The fractional change in the current because of the incident photons is therefore large and easily measurable if the photo-diode is reverse biased.