Question

The band gap of a semiconducting material is $\sim 2$ eV. Which one of the following absorption (A) vs. energy (in eV) curves is correct?

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

In semiconductors, absorption spectra exhibit a sharp rise at photon energies equal to the band gap $E_g$. Below $E_g$, absorption is negligible.

Answer 1

The Correct Option is D

Step 1: Recall band gap concept.
In semiconductors, the band gap ($E_g$) is the minimum energy required to excite an electron from the valence band to the conduction band.
- For photon energies $E<E_g$, photons cannot excite electrons across the gap → absorption is very low.
- For photon energies $E \geq E_g$, photons have sufficient energy → strong absorption begins.
Step 2: Apply given band gap.
Here $E_g \approx 2$ eV. Therefore: - For $E<2$ eV → absorption $\approx 0$ (or very small). - For $E \geq 2$ eV → absorption rises sharply and reaches high values ($\sim 90%$ or more).
Step 3: Check each option.
(A) Shows high absorption below 2 eV and drops afterwards → incorrect.
(B) Shows constant high absorption regardless of energy → incorrect.
(C) Shows constant low absorption regardless of energy → incorrect.
(D) Shows low absorption until 2 eV, then sharply increases → correct. Final Answer: \[ \boxed{(D)} \]