Energy Bands And Classification Of Solids
A materials technician measures the forbidden energy gap of an unknown crystalline solid and finds it to be close to 1.1 eV at ordinary room temperature. To which class of solids does this sample most probably belong?
Select the correct option:
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About This Question
- Subject
- physics
- Chapter
- semiconductor electronics
- Topic
- energy bands and classification of solids
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
Semiconductor
Solids are classified by the size of the energy gap (E_g) separating the top of the valence band from the bottom of the conduction band. In conductors the two bands overlap, giving (E_g \approx 0), so electrons move freely even at low temperature. In insulators the gap is very large, typically greater than 3 eV, so almost no electrons reach the conduction band at room temperature. Semiconductors occupy the intermediate range, with gaps of roughly 0.5 eV to 3 eV; silicon at 1.1 eV and germanium at 0.7 eV are the classic examples. A 1.1 eV gap is therefore the textbook signature of a semiconductor like silicon. The good-conductor option fails because conductors have overlapping or zero-gap bands. The perfect-insulator option fails because a 1.1 eV barrier is small enough for appreciable thermal excitation. The superconductor option is unrelated, since superconductivity is a low-temperature resistanceless state, not a band-gap classification. As a consistency check, a 1.1 eV gap corresponds to thermal accessibility (kT at 300 K is about 0.026 eV), allowing a small but useful carrier density, exactly the behaviour expected of a semiconductor.
This easy difficulty physics question is from the chapter semiconductor electronics, covering the topic of energy bands and classification of solids. It appeared in the 2025 exam.
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