What is the function of a transistor in an electronic circuit?

The TRANSISTOR (invented 1947, Bell Labs) is arguably the most important invention of the 20th century. TWO KEY FUNCTIONS: SWITCH: A small base current turns the collector-emitter current ON or OFF — this is how billions of transistors in microprocessors perform digital logic (0 or 1); AMPLIFIER: A small base signal controls a proportionally larger collector current — this is how audio amplifiers, radio receivers, and signal processing work. Modern CPUs contain billions of transistors on a single chip. Types: BJT (Bipolar Junction Transistor) — current-controlled; MOSFET (Metal-Oxide-Semiconductor FET) — voltage-controlled, used in digital circuits.