• $summary_{type}$: concept
• $word_{count}$: 170

Doping adds controlled impurities to dramatically increase conductivity. n-type: pentavalent dopant (P, As, Sb) substitutes for Si. Four of its five valence electrons form covalent bonds; the fifth is loosely bound (ionization energy ~0.05 eV). At room temperature, nearly all donors ionize: $n_e$ ≈ $N_D$ >> $n_i$. Donor level sits just below the conduction band. p-type: trivalent dopant (B, Al, Ga, In) has only three valence electrons — the fourth bond is incomplete, creating a hole (acceptor level just above valence band). $n_h$ ≈ $N_A$ >> $n_i$. Mass action law: $n_e$ * $n_h$ = $n_i^2$ always holds. Increasing majority carriers by doping proportionally decreases minority carriers. Both n-type and p-type are electrically neutral (dopant ions balance free carriers). Typical doping: 1 dopant per 10^6-10^8 Si atoms. At very high temperatures, intrinsic carriers dominate over dopant carriers — the semiconductor becomes effectively intrinsic again.