Solids are usually divided into metals and nonmetals. In a simplified scheme we may say that nonmetals consist of insulators and semiconductors. Both can be described by energy bands separated by a forbidden energy gap Eg• Whether a nonmetal is considered to be an insulator or a semiconductor often depends on the temperature at which its properties are to be investigated or used. In this chapter we use conductor to mean a nonmetal with a band gap from at least a few tenths of an eV up to a maximum of 10 eV. The considerable interest in semiconductors since the 1930s has been stimulated by their technical significance. Unlikl� metals, the physical properties of semi conductors can be considerably modified by introducing small amounts of foreign atoms. In this manner the resistivity of a silicon crystal can be changed by about seven orders of magnitude when one-millionth of the atoms in the crystal are replaced by suitable foreign atoms. Note that impurity substitution of this magni tude does not affect the band gap or other basic characteristics of the host material. Moreover, depending on the kind of foreign atom, the electrical current through the crystal is carried by electrons or holes. Hence multilayer structures with different types of conductivity can be constructed by doping a semiconductor crystal in homogeneously, resulting in devices such as diodes, transistors, lasers, etc. Experimentally it is found that replacing an atom of the host lattice by a foreign atom results in lattice defects with physical properties depending considerably on the particular atom introduced. Good understanding of such impurity centers has been achieved at least for Si and Ge when the foreign atom belongs to the groups of the periodic table closest to that of the semiconductor. They introduce localized donor and acceptor levels in the otherwise forbidden energy gap and are often described by the effective mass theory of Kahn & Luttinger (KL-EMT) (1-3), giving a hydrogen-like spectrum of levels with binding energies En' which can be written as