Why is photocurrent independent of reverse biased? I was recently reading about photodiodes, and the text said that the current (reverse bias) is independent of the reverse bias First of all, I didn't much understand the presence of a reverse bias, since current flow is due to incident light. Secondly, what must be the reason for no dependence on the reverse bias?
First of all, I didn't much understand the presence of a reverse bias, since current flow is due to incident light.
Reverse bias isn't required. A photodiode operated with reverse bias is said to be operated in photoconductive mode. A photodiode operated with 0 bias is said to be operated in photovoltaic mode. Photoconductive mode tends to produce the best responsivity and reduced junction capacitance. Photovoltaic mode tends to produce less dark current and thus less noise.
Secondly, what must be the reason for no dependence on the reverse bias?
Generally each photon received promotes one electron from the valence band to the conduction band in the depletion region. Thus it produces one electron and one hole, a pair of charge carriers. The photocurrent is, of course, proportional to the number of carriers.
If the absorption is strong enough, so that all incident photons are absorbed (each producing one carrier pair), then the photocurrent depends only on the photon flux of the incident light.
However, if the absorption is weak, then a secondary effect can occur. As the bias on the diode is changed, the length of the depletion region changes. More photons are absorbed as the depletion region volume increases. And so the response does, in this case, depend at least slightly on the bias.