CBSE Class 12 · 2006 · SemiconductorsShort Answer

An intrinsic semiconductor is a pure crystal (e.g. Si or Ge) in which conduction is only by thermally generated electron-hole pairs, so $n_e=n_h$.
(i) Doping with a pentavalent impurity (As, Sb, P) adds donor atoms whose extra electron occupies a donor level just below the conduction band; these electrons are easily excited to the conduction band, giving an n-type semiconductor with electrons as majority carriers.
(ii) Doping with a trivalent impurity (B, Al, In) adds acceptor atoms creating an acceptor level just above the valence band; valence electrons move into it leaving holes, giving a p-type semiconductor with holes as majority carriers.
$\boxed{\text{Pentavalent}\to \text{n-type};\text{trivalent}\to \text{p-type.}}$