Extrinsic (n-type & p-type)
Doping with pentavalent P (extra e⁻) or trivalent B (extra hole) — toggle type live.
Key Notes
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 ×.
Formulas
n-type (heavy doping)
N_D = donor concentration. Holes become minority.
p-type (heavy doping)
N_A = acceptor concentration. Electrons become minority.
Conductivity
Dominated by majority carriers.
Mass-action law
Universal — even when n and p are very different.
Important Points
Doping is the basis of all semiconductor devices — diodes, transistors, ICs.
Even 1 dopant per 10⁹ host atoms changes conductivity enormously.
n-type and p-type are both ELECTRICALLY NEUTRAL — the donor ion (+) balances the donated electron (−), and similarly for acceptors.
Doping levels typically 10¹⁵-10²⁰/cm³ — far below 10²² atoms/cm³ of the host.
At low T, dopant ionisation is partial (carrier freeze-out); at high T, n_i becomes comparable to N_D/N_A and intrinsic behaviour dominates.
Useful range of T for doped Si: ~150 K to ~450 K — beyond which intrinsic or freeze-out behaviour kicks in.
Extrinsic (n-type & p-type) notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.