A semiconductor transistor is a fundamental electronic component that acts as a switch or amplifier in circuits. It's made of semiconductor materials, usually silicon, and consists of three layers: emitter, base, and collector. The transistor controls the flow of current between the emitter and collector by modulating the smaller current at the base. As a switch, it can be turned on or off; as an amplifier, it boosts an input signal. 

Transistors IC revolutionized electronics, replacing bulkier vacuum tubes, enabling smaller, more efficient, and versatile devices like microprocessors, radios, and countless other digital and analog technologies. Their operation relies on the manipulation of the semiconductor's conductivity, which is altered by applying a voltage to the base.