presentation of the classical and quantum free electron theories and their successes and shortcomings (Chapter 23). In order to explain the large differences in the electrical properties of solids as well as the peculiar properties of semiconductors, the existence of allowed and forbidden energy bands is investigated (Chapter 24). In this chapter, we introduce the concepts of the electron effective mass and of holes. Intrinsic and doped semiconductors, their electron and hole densities, and their electrical properties are discussed in Chapter 25. It is now a rather simple matter for the...