emitterbase

The emitter-base junction refers to the p-n junction between the emitter and base regions in a bipolar junction transistor (BJT). In an NPN transistor, the emitter is heavily doped n-type, the base is p-type, and the collector is n-type; in a PNP transistor, the polarities are reversed. The emitter-base junction is the diode that, when forward biased, injects carriers from the emitter into the base.

In normal operation, the base-emitter junction is forward biased (approximately 0.6–0.7 volts for silicon, about 0.2–0.3 volts for germanium). This forward bias injects majority carriers from the emitter into the base, where the base is thin and lightly doped. Most of these carriers diffuse across the base and reach the reverse-biased base-collector junction, contributing to the collector current. The transistor action amplifies current, with the collector current Ic approximately proportional to the base current Ib (Ic ≈ β Ib, where β is the current gain). The total emitter current Ie equals Ib + Ic.

The emitter-base junction behaves as a diode and is central to transistor behavior. Its forward voltage drop, temperature sensitivity, and dynamic resistance influence device performance. The junction also has a reverse-bias breakdown limit (emitter-base breakdown, BV_EBO); exceeding this limit can permanently damage the transistor.

Understanding the emitter-base junction is essential for analyzing BJT modes of operation (active, saturation, and cutoff) and for modeling transistor behavior in analog and switching applications.