Carbon, silicon and germanium have four valence electrons each. At room temperature which one of the following statements is most appropriate? (a) The number of free electrons for conduction is significant only in Si and Ge but small in C. (b) The number of free conduction electrons is significant in C but small in Si and Ge. (c) The number of free conduction electrons is negligibly small in all the three. (d) The number of free electrons for conduction is significant in all the three

Answer: (a). Band gaps: C ≈ 5.4 eV, Si 1.1 eV, Ge 0.7 eV; kT ≈ 0.025 eV at room temperature. Small gaps of Si and Ge allow a significant number of electrons to be thermally excited; the large gap of C allows almost none. So the answer is (a).

Carbon, silicon and germanium have four valence electrons each. At… | JEE Main 2007 (AIEEE)

Q16JEE Main 2007 · AIEEEMedium

Energy Bands in Solids

Carbon, silicon and germanium have four valence electrons each. At room temperature which one of the following statements is most appropriate?

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Answer: (a)

Band gaps: C $\approx 5.4\text{ eV}$ , Si $1.1\text{ eV}$ , Ge $0.7\text{ eV}$ ; $kT \approx 0.025\text{ eV}$ at room temperature.
Small gaps of Si and Ge allow a significant number of electrons to be thermally excited; the large gap of C allows almost none.
So the answer is (a).

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