Question

The current in the forward bias is known to be more ($\text{in }mA$) than the current in the reverse bias ($\text{in }\mu A$ ). What is the reason, then, to operate the photodiode in reverse bias?

Answer 1

Remember the Application of Photodiode. The characteristics of a device forms the basis of the application of the device. Major applications are a direct result of the basic characteristics of the device.

Complete answer:
Photodiode is a PN Junction device which is majorly used as a photodetector. Photodiode is a semiconductor device which is used for the conversion of light energy into electrical energy (be it current or voltage). Photodiode is denoted by the symbol:

The construction of a Photodiode is very similar to that of a PN Junction device. And this device is mainly used in reverse biased condition.

Why Reverse Biased?
Photodiodes are generally used in reverse biased conditions due to a very simple reason. We know that under reverse biased conditions, in a PN Junction, current flows only due to minority charge carriers. When photons strike the depletion layer, they generate electron-hole pair for every incident photon. This generated pair further helps in conduction. Under reverse biased conditions, the fractional change in the current before and after the incidence of photon is significant. But in forward biased conditions, the fractional change in current, due to the generation of electron-hole pair, is negligible.
In photodiodes the change in current is important rather than the absolute value of current. Majorly, photodiodes are used in various sensors. And these sensors are required to be able to detect the change in parameter and not the absolute value of the parameter.
Thus, even with less current flowing under reverse biased condition ($\text{in }mA$), than as compared to forward biased condition ($\text{in }\mu A$), photodiode is operated under reverse biased condition as the fractional change in current under reverse biased condition is more than the forward biased condition.

Note:
The working of most of the electronic devices are considerably affected by their applications. Smoke Detectors, Photoconductors, Compact disc devices are few of the major applications of photodiode. Try to remember the important applications of such electronic devices.