Q46 / 180 · AIIMS 2010 · Chemistry

Assertion : Conductivity of silicon increases by doping it with group-15 elements. Reason : Doping means introduction of small amount of impurities like $\mathrm{P}, \mathrm{As}$ or Bi into the pure crystal.

  • A
    If both assertion and reason are true and reason is the correct explanation of assertion
  • Correct answer
    B
    If both assertion and reason are true but reason is not the correct explanation of assertion
  • C
    If assertion is true but reason is false
  • D
    If both assertion and reason are false.

Solution

When a silicon crystal is doped with a group-15 elements, such as P, As, Sb or Bi the structure of the crystal lattice is left unchanged but an occasional atom with five valence electrons occupies a site that would normally be occupied by a silicon atom. The foreign atom uses four of its electrons in covalent bonding but the remaining fifth electron becomes delocalised and is thus free to contribute to electrical conduction.

Hence, the correct answer is option: 'B'.

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