Practice question

**Energy bands in semiconductors** consist of valence band filled at 0 K and conduction band empty, gap E_g small ~1 eV (Si 1.1 eV, Ge 0.7 eV), insulators large gap >3 eV (C diamond 5.4 eV), conductors overlapping. Intrinsic semiconductor at 0 K behaves as insulator because no thermal excitation, at T>0 K electrons jump to conduction band leaving holes, conductivity increases with temperature. Metals have very low resistivity ( 10⁻² to 10⁻⁸ Ω m ) or high conductivity ( 10² to 10⁸ S m⁻¹ ) because of a large number of free electrons available for conduction. Substituting values gives Large number of free electrons,