From its early beginning before the war, the field of semiconductors has developped as a classical example where the standard approximations of 'band theory' can be safely used to study its interesting electronic properties. Thus in these covalent crystals, the electronic structure is only weakly coupled with the atomic vibrations; one-electron Bloch functions can be used and their energy bands can be accurately computed in the neighborhood of the energy gap between the valence and conduction bands; nand p doping can be obtained by introducing substitutional impurities which only introduce shallow donors and acceptors and can be studied by an effective-mass weak-scattering description. Yet, even at the beginning, it was known from luminescence studies that these simple concepts failed to describe the various 'deep levels' introduced near the middle of the energy gap by strong localized imperfections. These imperfections not only include some interstitial and many substitutional atoms, but also 'broken bonds' associated with surfaces and interfaces, dis location cores and 'vacancies', i.e., vacant iattice sites in the crystal. In all these cases, the electronic structure can be strongly correlated with the details of the atomic structure and the atomic motion. Because these 'deep levels' are strongly localised, electron-electron correlations can also playa significant role, and any weak perturbation treatment from the perfect crystal structure obviously fails. Thus, approximate 'strong coupling' techniques must often be used, in line' with a more chemical de scription of bonding.

Inhalt
Content.- 1. Atomic Configuration of Point Defects.- 1.1 Definition of Point Defects.- 1.2 Geometrical Configuration of Point Defects.- 1.3 Lattice Distortion and Relaxation.- 1.4 Defect Symmetry and Group Theory.- 1.5 Experimental Determination of Defect Symmetry.- 2. Effective Mass Theory.- 2.1 Simplified Presentation.- 2.2 Derivation in the One-Band Case.- 2.3 Pairing Effects.- 2.4 Experimental Observation of Shallow Levels.- 3. Simpte Theory of Deep Levels in Semiaonductors.- 3.1 The Elementary Tight-Binding Theory of Defects.- 3.2 Green's Function Theory of Defects: Tight Binding Application.- 4. Many-Electron Effects and Sophisticated Theories of Deep Levels.- 4.1 One-Electron Self-Consistent Calculations.- 4.2 Many-electron effects. The configuration interaction.- 5. Vibrational Properties and Entropy.- 5. 1 Vibrational Modes.- 5.2 Localized Modes Due to Defects.- 5.3 Experimental Determination of Vibrational Modes.- 5.4 Vibrational Entropy.- 6. Thermodynamics of Defects.- 6.1 Enthalpy of Formation.- 6.2 Defect Concentration at Thermal Equilibrium.- 6.3 On the Nature of the Defects Present at Thermal Equilibrium.- 6.4 Experimental Determination of Enthalpies.- 6.5 The Statistical Distribution of Donor-Acceptor Pairs.- 7. Defect Migration and Diffusion.- 7.1 Jump Probability and Migration Energy.- 7.2 Experimental Determination of Migration Enthalpies.- 7.3 Charge-State Effects on Defect Migration.- 7.4 Diffusion.- References.