When a small amount of a trivalent impurity like Boron, Indium, Aluminium or Gallium is added to a tetra valant element like Silicon or Germanium, three electrons of each impurity atoms forms a bond with the nearest silicon atom. The fourth bond remains incomplete which has a deficiency of one electron. This results in the creation of a hole. This hole attracts electrons from the neighbouring covalent bond. The impurity atom accepts electron from the semiconductor and becomes n-type semiconductor. In an ntype semiconductor the majority charge carriers are holes.