In the preceding chapters, and indeed in the subsequent chapters, most of the discussion is on semiconductors in which the Bloch theory of extended states prevails. There is another class of semiconductors that has received considerable attention over the past several decades, and that is disordered (or amorphous) semiconductors. Here, in the realm of nanostructures, we really do not want to discuss the entire field of amorphous semiconductors, and would generally ignore strongly disordered materials as well. However, recent experiments have shown the presence of a metal–insulator transition in quasi-two-dimensional systems. Consequently, one needs to understand the difference between localized (disordered) systems, weakly disordered systems, and the normal Bloch band picture of conductance.

Generally, in disordered (or, strongly localized) systems, the Boltzmann equation fails to describe transport adequately except under very special circumstances. Disordered materials can stem from several sources, ranging from amorphous materials to relatively good single crystals with very high doping concentrations. In particular, the latter exhibit a form of impurity-induced disorder when the concentration of the impurity reaches a significant fraction of the atomic concentration of the host lattice. This, in turns connects to weak localization which can also arise from impurity-induced coherence effects even when the concentration is not too high.