A bipolar junction transistor (BJT) is a three-terminal semiconductor device consisting of two PN junctions formed by the emitter, base, and collector regions. Based on the PN junction arrangement, it is classified into NPN and PNP types. Invented on December 23, 1947, by Drs. Bardeen, Brighton, and Shockley at Bell Labs, its core principle is to achieve amplification by controlling a larger change in the collector current through a small change in the base current. The internal doping concentration varies significantly: the emitter region is highly doped, the base region is the thinnest and least doped, and the collector region is the largest and moderately doped.
BJTs operate in three modes: cutoff, amplification, and saturation. Key parameters include the current amplification factor β (hFE), the characteristic frequency fT, and the collector-emitter breakdown voltage BUCEO. Modern BJTs are mostly made of silicon, and the collector current is changed by controlling the base-emitter voltage to alter the carrier diffusion in the emitter junction. As a fundamental component of electronic circuits, transistors possess signal amplification and electronic switching functions. They can be used to construct amplifiers to drive speakers and motors, or as switching elements in digital circuits and logic control. Typical applications include low-frequency/high-frequency power amplification and composite transistor designs.
