Question

Why a photo-diode is operated in reverse bias whereas the current in the forward bias is much larger than that in the reverse bias? Explain. Mention its two uses.

Answer 1

When a diode is reverse biassed and a small reverse current flows through it, the width of the depletion layer increases.

When light strikes the junction, a large number of electron-hole pairs are generated in the depletion layer due to the wide depletion layer. As a result, in reverse bias, the diode converts more incident light to current, i.e., photocurrent is greater in reverse bias than in forward bias.

Forward bias current is greater than reverse bias current, but the fractional change in reverse bias current is more easily measurable than the fractional change in forward bias current.

As a result, photo-diodes are used in reverse bias.

Uses: In smoke detectors, in bar code scanners.