Why are photodiodes preferably operated under reverse bias when the current in the forward bias is known to be more than that in reverse bias?
The two optoelectronic devices: - Photodiode and solar cell, have the same working principle but differ in terms of their process of operation. Explain the difference between the two devices in terms of (i) biasing, (ii) junction area and (iii) I-V characteristics.
Answers
Photodiodes convert incident light to electric current more effectively in reverse bias condition than in forward bias because the width of depletion region increases as you increase the applied reverse bias voltage across the diode (directly proportional) in a reverse biased pn junction. Hence, by applying a larger voltage, more of the incident photons are converted to electric current. However, when you forward bias a pn junction, the width of the depletion region reduces, so, only a small portion of the incident photons get converted to electric current.
i) Photodiodes are reverse biased (negatively biased), while solar panels are not biased at all (zero bias).
ii) The photodiode junction is larger than that of a solar panel (depletion region is larger for reverse biased junctions).
iii) The photodiode operates in the third quadrant; while the solar cell operates in the forth quadrant of the I-V characteristics (since one has a negative voltage bias, and the other is unbiased).
In reverse bias condition, width of depletion region increases which reduces the capacitance across the junction. Hence, the response time for absorption of photon increase which increase the sensitivity of photo diode which is required by it.