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. A p-type semiconductor is created by doping a tetravalent semiconductor (e.g., Si) with a trivalent impurity (e.g., B, Al, In), which accepts electrons and creates holes as majority carriers. Substituting values gives Trivalent atoms, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.