When a silicon crystal is doped with a group-15 element (with five valence electrons) such as P, As, Sb or Bi, the structure of the crystal lattice remains unchanged. Out of the five valence electrons of group-15 doped element, four electrons are used in normal covalent bonding with silicon and fifth electron is delocalised and thus conducts electricity. Doping a silicon crystal with a group-13 element (with three valence electrons) such as B, Al, Ga or in produces a semiconductor with three electrons in dopant. The place where fourth electron is missing is called as electron vacancy or hole.