What roles do the emitter, collector, and base serve in a transistor?

In a bipolar transistor, the emitter, base, and collector serve distinct roles in the conduction and control of electrical current.1

• Emitter: As the name implies, this terminal is responsible for emitting or injecting charge carriers (electrons or holes) into the device. It is typically heavily doped to facilitate this injection into the base region.1
• Base: This is the central, narrow region of the transistor. It acts as a control gate; a small input signal or voltage applied here modulates the flow of carriers through the device. By adjusting the base current, the transistor can amplify a signal or switch between "on" and "off" states.1234
• Collector: This terminal collects the charge carriers that have been injected by the emitter and have successfully passed through the base. Because the base-collector junction is typically reverse-biased, it maintains high resistance, allowing the device to control large output currents based on small input changes at the base.13