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. Doping increases majority carriers (electrons in n-type, holes in p-type), enhancing recombination with minority carriers, thus reducing their concentration indirectly. Substituting values gives Increased recombination, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.