When an acceptor is ionized a hole is freed, which requires an input
of energy. On the usual energy band diagram, an electron rises when it gains
energy, whereas a hole sinks in gaining energy.
Experimental ionization energies of acceptors in germanium and silicon
are given in Table 6. The Bohr model applies qualitatively for holes just as for
electrons, but the degeneracy at the top of the valence band complicates the
effective mass problem.
The tables show that donor and acceptor ionization energies in Si are comparable
with kBT at room temperature (26 meV), so that the thermal ionization
of donors and acceptors is important in the electrical conductivity of silicon at
room temperature. If donor atoms are present in considerably greater numbers
than acceptors, the thermal ionization of donors will release electrons
into the conduction band. The conductivity of the specimen then will be controlled
by electrons (negative charges), and the material is said to be n type.
If acceptors are dominant, holes will be released into the valence band
and the conductivity will be controlled by holes (positive charges): the material
is p type. The sign of the Hall voltage (6.53) is a rough test for n or p type.