Question: Q. 7. State the reason, why the photodiode is always operated under reverse bias. Write the working principle of operation of a photodiode. The semiconducting material used to fabricate a photodiode, has an energy gap of $1.2 \mathrm{eV}$. Using calculations, show whether it can detect light of wavelength of $400 \mathrm{~nm}$ incident on it.
U] [OD II Comptt., 2017]
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Solution:
Ans. Reason for use in reverse bias 1
Working Principle 1 Whether it can detectThe fractional change, due to photo effects, on the minority charge carrier dominates reverse bias current, which is much more than the fractional change in the forward bias current and can be easily detected Hence, photodiode is used in reverse bias.
Working principle of photodiode :
i. Generation of $e-h$ pairs due to light close to junction.
ii. Separation of electrons and holes due to electric field of the depletion region.
Detection is possible if $E_{p}>E_{g}$
$$ \begin{aligned} E_{p} & =\frac{h c}{\lambda} \mathrm{J} \ & =\frac{h c}{e \lambda} \mathrm{eV} \ & =\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{1.6 \times 10^{-19} \times 400 \times 10^{-9}} \ & =3.1 \mathrm{eV}\left(>E_{\mathrm{g}}\right) \end{aligned} $$
$1 / 2$
$\therefore$ It can detect this light.
[CBSE Marking Scheme, 2017]
