Question

Choose the correct statement. In semiconductors, the valence band and conduction band are

• A. separated by a large energy gap
• B. separated by a small energy gap
• C. almost empty
• D. overlapping each other

Hint:

To remember this classification easily, associate the band gaps with electrical behavior: Overlapping means electrons move completely freely (Conductors), a Large gap acts as an impassable wall (Insulators), and a Small gap allows partial or temperature-dependent electron passage (Semiconductors).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks us to identify the correct structural feature regarding the electronic energy band gap layout that characterizes a semiconductor material.

Step 2: Key Formula or Approach:
Solid materials are classified into three categories based on the size of their energy band gap ($E_g$) between the filled valence band and the empty conduction band: Insulators: Large forbidden energy gap ($E_g \gt 3\text{ eV}$). Semiconductors: Small forbidden energy gap ($E_g \lt 3\text{ eV}$). Conductors (Metals): Zero energy gap or overlapping bands ($E_g \approx 0\text{ eV}$).

Step 3: Detailed Explanation:
In a semiconductor crystal lattice at absolute zero temperature ($0\text{ K}$), the valence band is completely filled with bound electrons while the conduction band remains entirely empty, causing it to behave as an insulator. However, because the forbidden energy gap separating these two bands is small (typically around $1.1\text{ eV}$ for Silicon and $0.7\text{ eV}$ for Germanium), ambient thermal energy at room temperature is sufficient to excite a fraction of the valence electrons across the gap into the conduction band. These excited electrons become free charge carriers, allowing the material to conduct electricity under controlled conditions. This matches the description in statement (B).

Step 4: Final Answer:
The correct statement is that the bands are separated by a small energy gap, which corresponds to option (B).