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

Step-by-step 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.