Why are elemental dopants for Silicon or Germanium usually chosen from group XIII or group XV?
The size of the dopant atom should be equivalent to the size of Si or Ge. So that the symmetry of pure Si or Ge does not disturb and dopants can contribute the charge carrier on forming covalent bonds with Si or Germanium atoms. As the silicon and germanium belongs to XIV group so similar size of atom will be XIII and XV group of modern periodic table
