Question

A photodiode is operated in the reverse bias. Why?

• A. Photocurrent flows only in the reverse direction
• B. Thermally generated current is large in reverse bias as compared to the photocurrent
• C. Photocurrent is large in the forward bias as compared to the reverse bias current
• D. Photocurrent is significant in reverse bias as compared to the forward bias current

Answer 1

The correct option is D

Photocurrent is significant in reverse bias as compared to the forward bias current

Photodiode:

• The photodiode is a semiconductor device and it is also called a photo detector as it can convert optical signals into electrical signals.

Forward bias:

• When voltage is applied across a diode in such a way that the diode allows current, the diode is said to be forward-biased.

Reverse bias:

• In reverse bias, current flows due to minority charge carries and width of depletion region increases.

Why photodiode is operated in the reverse bias.

1. An electron-hole pair is created in the depletion layer when a $\mathrm{PN}$ junction is exposed to incident light, which causes the covalent bond to break as shown in the following figure.
2. Current flows across the junction as a result of the electric field pushing electrons to the $\mathrm{N}$ side and holes to the $\mathrm{P}$ side. The current is called as photocurrent and which depends on intensity of light falling on junction.
3. The width of the depletion region is narrower and slowly decreases when the voltage is elevated in a forward bias $\mathrm{PN}$ junction.
4. As a result, less current is generated in a narrow region where photons will break the bonds. In contrast, the width of the depletion region is greater and increasing as the voltage is increased in a reverse bias $\mathrm{PN}$ junction. Thus, there is more surface area for photons to react with, and a high current can be formed.
5. See the charge carriers generated are opposite charges they can easily recombine .So they have to be separated immediately after getting generated ,this is done by electric field or electric potential in the $\mathrm{PN}$ junction .
6. In the case of reverse bias, the junction potential is very large whereas low in forward condition.
7. Stronger fields allow for easier separation, which results in increased efficiency and continuous functioning.
8. So, it is conclude that, the photocurrent is significant in reverse bias as compared to the forward bias current.

Hence, option $\mathrm{D}$ is correct answer.