Which electrode of the bipolar transistor controls the output current?

A bipolar junction transistor (BJT) consists of three layers of semiconductor material corresponding to three terminals: the base, emitter, and collector. The fundamental operation of a BJT relies on a small current flowing into the base terminal to control a much larger current flowing between the collector and the emitter. Because the base current regulates the overall flow of electricity through the device, the base serves as the control electrode. The emitter emits the charge carriers and the collector collects them, but they do not control the main flow. The term "Source" is incorrect because it is a terminal found on a field-effect transistor (FET), not a BJT.