This chapter deals with the common types of transistors–the active devices used in electronic circuits mainly for signal amplification and switching. It analyzes the operation of an idealized device, based on physical principles, and derive its voltage–current characteristics and small-signal equivalent circuit. In contrast, the substrate is the base semiconductor that is common to all the transistors on the chip. The trend has been to decrease these dimensions as much as possible in order to pack a larger number of transistors per unit area of the silicon chip, and to increase switching speeds, as discussed later under short-channel effects. The small-signal equivalent circuit is the same for a PMOS transistor, in which the polarities of all currents and voltages are reversed. The insulating layer effectively isolates adjacent transistors, thereby reducing internal capacitances and leakage current. Complementary metal-oxide-semiconductor technology, which involves fabricating both NMOS and PMOS transistors on the same chip, is at present the dominant technology for digital integrated circuit.