1. The document discusses band theory and energy bands in solids, explaining that available electron energy states form bands rather than discrete energies as in atoms. 2. Materials are classified as conductors, insulators, or semiconductors based on their band structure, particularly whether the conduction and valence bands overlap or are separated by a band gap. 3. Semiconductors have a small band gap separating the almost-full valence band from the almost-empty conduction band, allowing electrical conductivity to be controlled by doping with impurities. Intrinsic semiconductors contain only the semiconductor material while extrinsic ones are doped with donor or acceptor atoms.