Question

There photo dlodes $D_1,D_2$ and $D_3$ are made of semiconductor having band gap $2.5eV,2eV$ and $3eV$ respectively. Which one will be able to detect light of wavelength $6000A^o$?

• A. $D_1$
• B. $D_2$
• C. $D_3$
• D. $D_1$ and $D_2$ both

Answer 1

Answer: (B)

$D_2$

Given that,

Band gap of $D_1=2.5eV$

Band gap of $D_2=2eV$

Band gap of $D_3=3eV$

Wave length $\lambda =6000\stackrel{\circ }{A}$

Now, the wave length for $2.5eV$

${\lambda }_1=\frac{hc}{E}$

${\lambda }_1=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{2.5\times 1.6\times {10}^{-19}}$

${\lambda }_1=4.95\times {10}^{-7}m$

Now, for $2eV$

${\lambda }_2=\frac{hc}{E}$

${\lambda }_2=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{2\times 1.6\times {10}^{-19}}$

${\lambda }_2=6.2\times {10}^{-7}m$

Now, for $3eV$

${\lambda }_3=\frac{hc}{E}$

${\lambda }_3=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{3\times 1.6\times {10}^{-19}}$

${\lambda }_3=4.13\times {10}^{-7}m$

For detection of optical signal the wave length of incident energy radiation must be greater.

So, only $D_2$can detect the radiation