Question

In a semiconductor, the energy gap between the valence band and the conduction band is nearly:

• A. 5 eV
• B. 1 eV
• C. 15 eV
• D. zero

Hint:

Semiconductors have a small forbidden gap near 1 eV; metals have zero and insulators have a large gap of several eV.

Answer 1

The Correct Option is B

Step 1: Recall band classification.
Solids are grouped by the forbidden energy gap \( E_g \) between the top of the valence band and the bottom of the conduction band:
- Conductors: valence and conduction bands overlap, \( E_g \approx 0 \).
- Semiconductors: a small gap, \( E_g \approx 1 \) eV.
- Insulators: a large gap, \( E_g > 3 \) eV (often \( \approx 5 \) eV or more).

Step 2: Use standard values.
For the common semiconductors, \[ E_g(\text{Si}) \approx 1.1\ \text{eV},\qquad E_g(\text{Ge}) \approx 0.7\ \text{eV}. \] Both are close to 1 eV.

Step 3: Match to the options.
The value nearest these is 1 eV.

Step 4: Reject the others.
5 eV corresponds to an insulator like diamond; 15 eV is unphysically large for a crystal band gap; zero corresponds to a metal. Hence option (ii) 1 eV is correct.
\[\boxed{E_g \approx 1\ \text{eV}}\]