This book encapsulates the fundamental quantum processes of importance in the physics and technology of semiconductors in a relatively informal style that has been found to be attractive for graduate courses. This fourth edition is expanded by the addition of new chapters on quantum transport, semi-classical transport and space-charge waves, extending the discussion to statistical, many-particle behaviour in transport phenomena. The author has also taken the opportunity to update other sections. As with previous editions, the text restricts its attention to bulk semiconductors. The account traces the path from quantum processes describable by density matrices, through the semi-classical Boltzmann equation and its solutions, to the drift-diffusion description of space-charge waves, the latter appearing in the contexts of negative differential resistance, acoustoelectric and recombination instabilities. Besides being a useful reference for workers in the field, this book will be a valuable text for graduate courses.