Publisher Summary The chapter discusses various pieces and parts that make up an electrical circuit. It starts with semiconductors, which it defines as a material that partially conducts electricity or a material that is only semigood at conducting electricity. The chapter then goes on to discuss various semiconductor devices used in circuits such as diodes and transistors and their different modes of operation. Further, the chapter discusses field effect transistors (FETs) that have some properties that make them very desirable components. The primary reason they are so efficient is that the output of a FET is a resistance that varies depending on the voltage at the input. Next, the chapter describes miscellaneous components such as Darlington transistor, silicon controlled rectifier (SCR), TRIAC, and isolated gate bipolar transistor (IGBT). The chapter further explains the concepts of heat and power management. Finally, the operational amplifiers, positive, and negative feedbacks are described while the popular microcontrollers and microprocessors and their inputs and outputs are also explained.