Consider now the case when a small concentration of group III atoms is added to the host Si crystal at the manufacturing stage. These atoms will also substitute for Si atoms in the lattice. Since they have three rather than four valence electrons, one of the four covalent bonds associated with the impurity is unfilled. At 0 K an unfilled bond remains in the vicinity of the impurity atom through Coulombic attraction between the negative charge acquired by the impurity on completing its complement of four valence bonds and the subsequent appearance of a positive charge on a nearby Si atom which must sacrifice an electron from one of its covalent bonds. Rather than thinking in terms of unfilled covalent bonds, it is usual to treat the problem as one of a positive charge, called a hole , orbiting the negatively charged acceptor impurity to form a hydrogenic-like system. The two alternative pictures are shown in figure 9. Again, the radius of the hole orbit is large, extending over many unit cells. As a consequence, the binding energy is low and only a small amount of "external" energy is required to liberate the hole from its acceptor impurity atom. Therefore, the hole becomes a free, valence band hole at relatively low temperatures as depicted in figure 10. Note that the release of the hole by the acceptor impurity does NOT result in the generation of an electron. Rather, the acceptor becomes ionised (negative charge) but is locked into the lattice and unable to move. It too can influence the motion of charge when an electric field is applied to the semiconductor and a current flows. Generally, it can be assumed that the hole concentration at room temperature in p -type semiconductor is just equal to the total concentration of all acceptors since the overwhelming majority will be ionised;
and the concentration of acceptors which remain neutral (un-ionised) at 300 K , N 0 A = 0
Figure 9: Silicon at T = 0K containing a trace concentration of group III impurity atoms. The hole is bound to the impurity atoms. The hole is bound to the impurity by Coulombic attraction.
Figure 10: Silicon at T>0K, with group III impurities ionised and free holes available for conduction.