Why do photodiodes operate in reverse bias?

Photodiodes more effectively convert incident light into electrical current in the off-state than in the off-state, as the width of the stopband increases as you increase the blocking voltage – directly proportional applied to the diode in a reverse-biased pn junction.

By applying a larger voltage, therefore, more of the incident photons are converted into electric current. However, if you bias a pn junction in the forward direction, the width of the depletion region decreases, so that only a small portion of the incident photons is converted into electrical current.

  • Photodiodes are reverse biased (negatively biased) while solar modules are not biased at all (zero bias).
  • The photodiode junction is larger than that of a solar panel (depletion region is larger for reverse biased junctions).
  • The photodiode operates in the third quadrant; while the solar cell is working in the fourth quadrant of the I-V characteristic (because it has a negative bias and the other is not biased).

Another way to look at the reverse biased PN junction is a high field region (due to the dopant ions remaining through the reverse biased junctions). The moment you create a hole-electron pair from a photon, each of these charge carriers is rapidly separated and swept out of the depletion region to produce a photocurrent. This is directly analogous to the reason why a base collector in a bipolar transistor is reverse biased in the forward linear boost mode.

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