Extrinsic (n-type & p-type)

Extrinsic semiconductor = intrinsic + dopants. Two types:

n-type: doped with Group V element (P, As, Sb) — has one EXTRA valence electron ⇒ donates to CB.

Donor level is just below CB. Even at low T, donors ionise easily ⇒ many free electrons.

p-type: doped with Group III (B, Ga, Al) — has one FEWER valence electron ⇒ creates HOLES in VB.

Acceptor level is just above VB. Electrons easily jump from VB to acceptor ⇒ many free holes.

Majority carriers: n-type → electrons; p-type → holes. Minority carriers are the opposite.

Mass-action law: n·p = n_i² always holds. In doped: one carrier is much larger than n_i, the other much smaller.

Doping concentrations are tiny (1 part per million or less) but change conductivity by ~10⁶ ×.