HOLE

in solid-state physics, a phenomenon produced by the movement of valence electrons through certain metals and semiconductors, resulting in the creation of a vacancy in a corresponding energy state. Each vacancy, or hole, represents the absence of an electron from an accessible energy state in the solid and, thus, a centre of positive charge. According to the band theory of solids, electrons within a solid have energies at only certain discrete levels. These discrete levels combine to form groups of energy levels, or bands. The band behaves as though each level from which an electron was removed contains a positive charge, or hole. Holes are often deliberately introduced into a solid in order to increase its ability to carry an electric current. Through a process called doping, small quantities of impurity atoms are added to an otherwise pure semiconductor. In p-type semiconductors the impurity atoms contain fewer valence electrons than the atoms in the solid. A p-type semiconductor can be formed by the introduction of boron (B) atoms into a crystal of germanium (Ge). A boron atom contains three valence electrons and a germanium electron has four valence electrons, so every boron atom added to the semiconductor creates a deficiency of one electron as well as a hole of positive charge. This hole must then be filled by the movement of another electron from within the crystal, which, in turn, creates a second hole. As electrons fill some holes while creating others, the resulting movement of positive charge encourages conduction. See also band theory.