Energy Band Diagram

In a solid, atomic energy levels broaden into BANDS due to overlap of electron wavefunctions.

Valence band: highest filled (or partially filled) band at T = 0. Conduction band: next higher (mostly empty).

Energy gap E_g = bottom of conduction band − top of valence band.

Conductors: bands overlap or valence band partially filled ⇒ free electrons readily move. Eg ≈ 0.

Insulators: large gap (E_g > ~3 eV). Few electrons can be thermally excited to the CB.

Semiconductors: small gap (E_g ~ 0.5-2 eV). Some thermal excitation at room temperature.

Conduction in semiconductors involves BOTH electrons in CB and HOLES (missing electrons in VB) — holes act like positive charge carriers.

Doping creates extra states INSIDE the gap close to CB (donor → n-type) or VB (acceptor → p-type), dramatically changing conductivity.