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Q)

A p-n photo diode is fabricated from a semi - conductor with a band gap of $2.5 \;ev$. It can detect a signal of wavelength

$\begin{array}{1 1} 4000\;nm \\ 6000\;nm \\ 4000^{\circ}A \\ 6000^{\circ}A \end{array}$

$E_g=2.5\;ev$
$\qquad= \large\frac{hc}{\lambda}$
$\qquad= \large\frac{2400 \;ev^{\circ}A}{\lambda}$
$\lambda= \large\frac{12400\;ev A^{\circ}}{2.5 eV}$$=4960\;A^{\circ}$
The radiation of wave length $\lambda= 4000 A^{\circ}$ can excite this