PYQ NEET- Semiconductor; Devices And Simple Circuits L-5

Question: An LED is constructed from a p-n junction diode using GaAsP. The energy gap is $1.9 \mathrm{eV}$. The wavelength of the light emitted will be equal to#

A) $10.4 \times 10^{-26} \mathrm{~m}$

B) $654 \mathrm{~nm}$

C) $654 \AA$

D) $654 \times 10^{-11} \mathrm{~m}$

Answer: $654 \mathrm{~nm}$#

Sol:#

The energy of light of wavelength $\lambda$ is given by $$ E=h v=\frac{h c}{\lambda} \Rightarrow \lambda=\frac{h c}{E} $$

Here,$h=$ Planck’s constant $$ \begin{gathered} =6.63 \times 10^{-34} \mathrm{~J}-\mathrm{s} \ c=\text { speed of light }=3 \times 10^8 \mathrm{~m} / \mathrm{s} \ E=\text { energy gap }=1.9 \mathrm{eV}=1.9 \times 1.6 \times 10^{-19} \mathrm{~J} \end{gathered} $$

Substituting the given values in Eq. (i), we get $$ \begin{aligned} \Rightarrow \quad \lambda & =\frac{6.63 \times 10^{-34} \times 3 \times 10^8}{1.9 \times 1.6 \times 10^{-19}} \ & =6.54 \times 10^{-7} \mathrm{~m} \approx 654 \mathrm{~nm} \end{aligned} $$

Thus, the wavelength of light emitted from LED will be $654 \mathrm{~nm}$.