Energy Bands And Classification Of Solids

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.