A fundamental knowledge of solid-state physics is used to gain understanding of the two basic active solidstate devices-bipolar and field-effect transistors. The bipolar transistor is treated as two junctions communicating through a common region (the base). The functions of charge injection and collection are studied and developed to explain the observed static (d-c) volt-ampere characteristics. Major bipolar-transistor parameters are then reviewed. Finally, the limitations in gain, voltage, and power are discussed. There are basically two forms of field-effect transistors: the junction field effect transistor (JFET) and the metal oxide semiconductor (MOS). Both forms use a voltage applied to a gate electrode or region to control charge (current) flow between source and drain terminals. This action is studied and used to to develop the observed state (d-c) volt-ampere characteristic. The major field-effect transistor parameters are then reviewed. Finally, the limitations in gain, voltage, and power are discussed. Included in the appendixes is a brief review of transistor evolution, which serves as an introduction to device processing information, and an introduction to some of the lesser known solid-state devices. A third appendix explains thermal resistance.