With what considerations in view is a photodiode fabricated? State its working with the help of a suitable diagram. Even though the current in forward bias is known to be more than in reverse bias, the photodiode works in reverse bias — what is the reason?
Solution
Fabrication considerations: the junction is made with a transparent window so that light reaches the depletion region, and the material's band gap is chosen so that the photons to be detected have hν>Eg. Working: in reverse bias, incident photons generate electron-hole pairs in the depletion region; these are swept across the junction, producing a reverse photocurrent proportional to the incident light intensity. Reason for reverse bias: on illumination the fractional change in the small reverse (minority-carrier) current is large and easily measured, whereas the already large forward current changes negligibly. Iphoto∝intensity; reverse bias gives measurable change.
