In metals: valence and conduction bands overlap — free electron flow. In insulators: large band gap (> 3 eV, e.g., diamond 5.5 eV). In semiconductors: small band gap (~1 eV, Si = 1.1 eV, Ge = 0.67 eV). Intrinsic semiconductors: pure Si/Ge, conductivity increases with temperature (more electrons jump gap). Extrinsic semiconductors: n-type (doped with Group 15: P, As, Sb — extra electrons, majority carriers are electrons) and p-type (doped with Group 13: B, Al, Ga — electron holes, majority carriers are holes). p-n junction forms a diode.