When a piece of pure silicon is doped with aluminium, then
- A.the conductivity of the doped silicon piece will remain the same
- B.the doped silicon piece will become n-type
- C.the doped silicon piece will become p-type
- D.the resistivity of the doped silicon piece will increase
Correct answer
Explanation
The correct answer is C, the doped silicon piece will become p-type. Aluminium has three valence electrons, one fewer than silicon, so each aluminium atom leaves a hole that acts as a positive charge carrier.
Silicon has four valence electrons and forms four covalent bonds with its neighbours. When a trivalent impurity such as aluminium, boron, gallium or indium takes the place of a silicon atom, one bond is left incomplete, creating a hole. These impurities are called acceptors. Holes become the majority carriers, so the material is a p-type semiconductor. Adding a pentavalent impurity such as phosphorus or arsenic gives extra electrons instead and makes n-type silicon.
Option A is wrong because doping greatly increases conductivity. Option B is wrong because n-type needs a pentavalent donor, not aluminium. Option D is wrong because more charge carriers lower the resistivity, not raise it.
Exam tip: trivalent (B, Al, Ga, In) gives p-type, holes; pentavalent (P, As, Sb) gives n-type, electrons.
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