Q59 / 180 · AIIMS 2015 · Chemistry

Assertion : Conductivity of silicon increases by doping it with group-15 elements. Reason : Doping means introduction of small amount of impurities like $\mathrm{P}$, 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
    Ifboth 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 element like P, As, Sb or Bi, the silicon atoms at some lattice sites are substituted by atoms of group-15 element. Asthese atoms have 5 electrons in the valence shell, after forming normal four covalent bonds with silicon atoms, the extra fifth electron is free and gets delocalized and hence, increases the conductivity of silicon.

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

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