What is a transistor?

• A. A semiconductor device comprised of an npn or pnp junction with a source, gate, and drain or emitter, base, and collector. A transistor can act as an electrical switch which can turn on and off thousands of times a second. They are used in opamps for signal amplification as well as in digital circuits.
• B. A passive resistor used to limit current
• C. A coil used to store energy
• D. A diode used for rectification

Answer: (A)

A transistor is a three-terminal semiconductor device that can control a larger current with a smaller one, enabling both amplification and fast switching. There are two main families: bipolar junction transistors (BJT) and field-effect transistors (FET). In a BJT, a small current into the base modulates a larger current from emitter to collector; in a MOSFET, a voltage at the gate controls the current between source and drain. The description given highlights a BJT structure with npn or pnp configurations and the classic emitter-base-collector terminals, noting its ability to turn on and off quickly and to amplify signals in op-amps or drive digital circuits. This combination of active behavior—providing gain and switching capability—defines what a transistor is. By contrast, a resistor, an inductor, or a diode are passive or serve different functions and do not provide the same switching and amplification role.