Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2019 / Paper 2 / Section B / 21 / ii / Solution

n-type doping adds donor impurities with weakly bound electrons near the conduction-band edge; ionization supplies conduction electrons and leaves positive donor ions. p-type doping introduces acceptors near the valence-band edge, producing holes and negatively charged ionized acceptors. Silicon and germanium commonly use group-V donors and group-III acceptors; dopant behaviour in a compound semiconductor depends on its lattice site.

Doping sets carrier concentration, Fermi level and resistivity. Spatially patterned donor and acceptor regions form p-n junctions, transistor source/drain regions and electrical contacts; remote doping can supply carriers to a cleaner conducting layer. Compensation and incomplete ionization mean carrier density need not equal the total impurity concentration.