Question

A pn photodiode is fabricated from a semiconductor with a band gap of $2.5eV$ It can detect a signal of wavelength:
A. $4960{A^o}$
B. $6000{A^o}$
C. $4000nm$
D. $6000nm$

Answer 1

You have given the value of band gap. Use the formula of Wavelength and substitute the value of band gap to detect a signal of wavelength. The second hint is considering the signals having wavelength < threshold wavelength.

Complete step by step answer:
Photodiode- It is a device that can be used to convert light into an electrical current. When photons are absorbed in the photodiode, then the current is produced. Photodiode is commonly known as photosensor or photodetector as well.
The operation of Photodiode can be of two types:
Photovoltaic mode: This device does not provide the external reverse potential and that is why it is also known as zero bias modes.
Photoconductive mode: Sometimes it can behave like a photoconductive device. In this condition, there is certain reverse potential is provided to the device.
According to the question the band gap = $2.5eV$
Wavelength of radiation corresponding to this energy,
$\lambda = \dfrac{{hc}}{E}$
$\implies$ $\lambda = \dfrac{{12400}}{{2.5}}$
$\therefore$ $\lambda = 4960{A^o}$.

So, the correct answer is “Option A”.

Note:
A band gap is the distance between the valence band of electrons and the conduction band. Essentially, the band gap represents the minimum energy that is required to excite an electron up to a state in the conduction band where it can participate in conduction.
List of some applications of Photodiode are as follows:
Mainly used in counters and switching circuits.
Highly useful in an optical communication system.
Photodiode is also used in logic circuits and encoders.