Those solids which have intermediate conductivities ranging from10‌–6 to 10‌4ohm‌–1 m‌–1 are classified as semiconductors. As the temperature rises, there is a rise in conductivity value because electrons from the valence band jump to conduction band.
(i) n-type semiconductor : When a silicon or germanium crystal is doped with group 15 element like P or As, the dopant atom forms four covalent bonds like Si or Ge atom but the fifth electron, not used in bonding, becomes delocalised and continues its share towards electrical conduction. Thus silicon or germanium doped with P or As is called n-type semiconductor, n-indicative of negative since it is the electron that conducts electricity.
(ii) p-type semiconductor : When a silicon or germanium is doped with group 13 element like B or Al, the dopant is present only with three valence electrons. An electron vacancy or a hole is created at the place of missing fourth electron. Here, this hole moves throughout the crystal like a positive charge giving rise to electrical conductivity. Thus Si or Ge doped with B or Al is called p-type semiconductor, p stands for positive hole, since it is the positive hole that is responsible for conduction.