In a bipolar junction transistor (BJT), what are the three terminals called?

• A. Emitter, Base, Collector
• B. Source, Gate, Drain
• C. Anode, Cathode, Gate
• D. Input, Output, Ground

Answer: (A)

The three terminals are emitter, base, and collector. In a BJT, the emitter is the region that injects carriers (electrons or holes) into the base. The base is a thin, lightly doped region that controls the flow of these carriers. The collector collects the carriers that have crossed the base, allowing current to flow from the collector to the emitter. In normal active operation, the base-emitter junction is forward-biased while the base-collector junction is reverse-biased, so a small current into the base controls a much larger current from the collector. This is the basis of current amplification in BJTs. The other names listed belong to different devices (for example, source/gate/drain are for MOSFETs; anode/cathode/gate relate to diodes/valves), so they don’t describe a BJT’s terminals.