Practice question

**Number of carriers** from doping: Ge crystal 4×10²⁸ atoms/m³ doped 2 ppm trivalent gives acceptor atoms 8×10²² m⁻³, holes ≈ that, for 1.5 ppm 6×10²² m⁻³, for 0.5 ppm pentavalent Si 5×10²⁸ atoms/m³ gives 2.5×10²² donors/m³. Acceptor atom effectively negative when accepts electron, donor positive when donates, but crystal neutral. The outer orbit of Si (third shell) can hold 8 electrons (2s + 6p), though Si has only 4 valence electrons naturally forming covalent bonds. Substituting values gives 8, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.