Practice question

**Doping** is adding impurity to pure semiconductor to increase carriers, pentavalent (P, As, Sb) donates extra electron, 1 ppm doping in Ge with 4×10²⁸ atoms/m³ gives donor density N_d = 4×10²⁸×10⁻⁶ =4×10²² m⁻³ for 1 ppm, acceptor atoms p-type trivalent B, Al, Ga. Overall charge neutrality maintained because donor ion core positive but electron negative, net neutral. Trivalent acceptor impurities (e.g., B, Al) in a p-type semiconductor lack one electron per atom, creating holes that act as majority carriers by accepting electrons from the lattice. Substituting values gives Holes, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.