For example, when Silicon (Si), having four valence electrons, needs to be doped as an p-type semiconductor, elements from group III like Boron (B) or Aluminium (Al) have three valence electrons. When substituting a B atom in the crystalline lattice the three valence electrons form covalent bonds with three of the Si neighbours but the bond with the fourth neighbour remains unsatisfied. The unsatisfied bond attracts electrons from the neighbouring bonds. At room temperature always an electron from a neighbouring bond will jump to repair the unsatisfied bond leaving a hole which can move around the crystal and carry a current. The initially electroneutral acceptor becomes negatively charged (ionised).