BJTs

BJTs, or bipolar junction transistors, are semiconductor devices that use both electron and hole charge carriers to amplify or switch electronic signals. A BJT has three regions—emitter, base, and collector—forming two p-n junctions. The emitter is heavily doped, the base is thin and lightly doped, and the collector is moderately doped. The device amplifies current, with the collector current Ic approximately proportional to the base current Ib, characterized by the current gain beta (or hFE) in active operation.

BJTs come in two polarities: NPN and PNP. In an NPN transistor, electrons are the majority carriers; in a PNP transistor, holes are. In forward-active operation, the base-emitter junction is forward biased and the base-collector junction is reverse biased, allowing a controlled collector current. In saturation both junctions are forward biased, limiting Ic, while in cutoff both are reverse biased, resulting in near-zero Ic.

Common configurations include common-emitter, common-base, and common-collector. The common-emitter configuration provides substantial voltage and current gain and inverts the signal; the common-base configuration has low input impedance and high output impedance with no phase inversion; the common-collector configuration (emitter follower) offers high input impedance and low output impedance, with no voltage gain but high current gain.

Key parameters include beta (current gain), Vce (collector-emitter voltage), Vbe (base-emitter voltage), and fT (transition frequency). Performance is temperature dependent, with beta and leakage currents varying as temperature changes. Applications span analog amplification, switching in digital logic, and power regulation, making BJTs a foundational component in many electronic circuits.