Q1(c). Conductors, semiconductors and insulators: band picture

Answer in brief

The difference is the gap Eg between the filled valence band and the conduction band: zero in conductors, small (about 0.1 to 3 eV) in semiconductors, large (more than 3 to 4 eV) in insulators.

Key points

• Overlap of atomic orbitals of N atoms gives a band of N closely spaced levels; the filled valence band and empty conduction band are separated by the gap Eg.
• Current flows only if electrons sit in a partly filled band, where empty levels lie just above them.
• Electrons cross a gap only by thermal excitation: kT = 0.026 eV at 300 K, so only a small gap gives thermal carriers.
• Conductor: σ falls on heating; semiconductor: σ rises steeply, n ∝ e^−Eg/2kT; insulator: stays negligible.
• Doping a semiconductor (group 15 donor: n-type; group 13 acceptor: p-type) raises σ without changing Eg.