If a trivalent impurity is mixed in a pure (intrinsic) semiconductor, then it becomes a p-type semiconductor. As given in , indium is trivalent impurity so it must be doped to Ge (or Sr) to make p-type semiconductor. ⇒ The added impurity is very small of the order of 1 atom per million atoms of the pure semiconductor when impurity atoms are in corporate in a pure semiconductor crystal, the electric properties are altered and most of the charge carriers originate from impurity atoms.