Question

Assertion: A p-n junction with reverse bias can be used as a photo-diode to measure light intensity.
Reason: In a reverse-bias condition, the current is small but is more sensitive to changes in the incident light intensity.
A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C) Assertion is correct but Reason is incorrect.
D) Assertion is incorrect but the reason is correct.

Answer 1

Hint: You must already be aware that the boundary or the separation interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor is known as the p-n junction whereas biasing is the application of a known voltage across a p-n junction; In reverse biasing, the n-type semiconductor material is connected to the positive terminal of the battery and the p-type semiconductor material is connected to the negative terminal.

Complete solution:
A photodiode is a semiconductor device that converts light into an electrical current; the current is generated when photons fall on the surface of the semiconductor and are absorbed in the photodiode. When the diode is reverse-biased, the current only flows through the photodiode with incident light creating photocurrent; hence we can say that our Assertion statement is correct. The reverse bias causes an increase in the potential across the depletion region and an increase in the width of the depletion region, thus leading to an increase in the surface area which helps to absorb the maximum amount of photons. Hence in a reverse-bias condition, the current is small but more sensitive to changes in the intensity of light incident on the diode. Therefore, our Reason is correct and also the correct explanation for the Assertion.

Hence option (A) is the correct answer.

Note: There is an added benefit to using reverse-bias condition. In reverse-bias conditions, there is a linear output of the photodiode with respect to the illumination. This means that the voltage and current change linearly (directly proportional) with increasing optical power. Also, a photodiode can be used without applying any voltage at all; this is usually done when the application depends on extremely low noise and low dark current.