Signal-Level Transistors

A transistor is a three-terminal semiconductor device capable of amplification and switching. It is essentially the solid-state analogy of the triode vacuum tube. There are two principal classes of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs). These transistors are made as discrete small-signal and power devices. Variations of them are integrated into digital and analog or linear ICs. Small-signal discrete BJTs remain popular in low-frequency circuits, while small-signal discrete FETs meet the requirements for high-input impedance transistors. Discrete power BJTs are still popular in low-frequency and linear circuits, but discrete metal-oxide semiconductor (MOSFET) transistors are preferred for high-frequency switching.