The addition of a trivalent atom (e.g. boron) to silicon leads to an empty
electron state, or positive hole, which can be ionized from the effective single
negative charge e on the B atom. The ionization energy is again about 0.01 eV,
as might be expected. Therefore the doping of silicon with boron leads to the introduction of acceptor states about 0.01 eV above the top of the valence band,
as shown in Fig. 2.11, and to p-type semiconductivity. In the case of n- or p-type
semiconductivity the temperature dependence of the conductivity is similar to
that in Eq. (2.37) with Eg replaced by Ei.