A nonmathematical description of the basic ideas and concepts underlying the field of semiconductors and semiconductor devices is given here. The role of the energy band scheme, of the impurity donor and acceptor atoms, and of thep-njunction in semiconductor device physics is discussed in detail. The nonsymmetrical conduction or rectification properties of thep-njunction, which determine such things as forward and reverse voltage-current relationships, are qualitatively treated. Thep-njunction theory is then extended to the physics of transistor action and to a discussion of the fundamental parameters which enter into transistor behavior.