Conductor vs Insulator vs Semiconductor

CLASSIFICATION of solids by electrical conductivity / band structure:

Conductors (metals): partially filled or overlapping bands; resistivity ~10⁻⁸ Ω·m; conductivity DECREASES with temperature.

Insulators: large bandgap (E_g > 3 eV); resistivity 10⁸-10¹⁶ Ω·m; essentially no free carriers at room temperature.

Semiconductors: small bandgap (0.5-2 eV); resistivity 10⁻⁴-10⁴ Ω·m; conductivity INCREASES with temperature.

Conductivity range spans ~24 orders of magnitude across these classes — most extreme variation in all of solid-state physics.

Doping makes semiconductors functionally indispensable — controllable conductivity by orders of magnitude.

Examples: Cu, Ag, Au (conductors); diamond, rubber, glass (insulators); Si, Ge, GaAs (semiconductors).