When a piece of pure silicon is doped with aluminium, then

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the conductivity of the doped silicon piece will remain the same

the doped silicon piece will become n-type

the doped silicon piece will become p-type

the resistivity of the doped silicon piece will increase

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This question was previously asked in

UPSC CDS-1 – 2017

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The correct answer is C) the doped silicon piece will become p-type.

Pure silicon is a semiconductor with a valence of four. Aluminium is a Group 13 element and acts as a trivalent impurity (valence of three) when doped into silicon. When a trivalent impurity replaces a silicon atom in the crystal lattice, it creates a ‘hole’ (absence of an electron) in the covalent bond structure. These holes act as positive charge carriers, making the doped semiconductor p-type.

Doping semiconductors with impurities increases their conductivity. Doping with trivalent impurities creates p-type semiconductors (majority carriers are holes), while doping with pentavalent impurities (like Phosphorus or Arsenic from Group 15) creates n-type semiconductors (majority carriers are electrons). Resistivity is the inverse of conductivity, so doping decreases resistivity.

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