UPSC Combined Geo-Scientist, Mains · 2017 · Mains Paper I

Q1(b). Phosphorus-doped silicon

Question

When a few Si atoms are replaced by P atoms in pure crystal of Si, what is the consequence in the physical property?

Phosphorus produces an n-type semiconductor: the electrical conductivity rises because electrons become the majority carriers.

Silicon forms four covalent bonds in its tetrahedral lattice.
Phosphorus introduces a donor level just below the conduction-band edge. Thermal energy readily promotes the donor electron into the conduction band.
Ionised phosphorus donors remain fixed in the lattice.

The full model answer

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