• 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.