Question

A $\text{p - n}$ photodiode is fabricated from a semiconductor with band gap of $2.8\text{eV}$. Which of the following wavelengths cannot be detected ?

• A. $6000\text{nm}$
• B. $60\text{nm}$
• C. $420\text{nm}$
• D. $100\text{nm}$

Answer 1

The correct option is A $6000\text{nm}$

$\text{Band gap}=2.8\text{eV}$

$E=2.8\text{eV}$

$\frac{12400}{\lambda \left(A\right)}=2.8\text{eV}$

$\lambda =\frac{12400}{2.8}=4428.57A^{\circ }=442.8\text{nm}$

This is the ${\lambda }_{max}$ for given band gap.

$\therefore 100\text{nm},60\text{nm}\&420\text{nm}$ can be detected.

whereas , $6000\text{nm}>442.8nm$ $\therefore$ cannot be detected.