BJTn

BJTn refers to a bipolar junction transistor of the NPN type, one of the two complementary BJTs (the other being PNP). It is a three-terminal semiconductor device used for amplification and switching in electronic circuits.

Construction: It comprises three doped regions: an n-type emitter, a p-type base, and an n-type collector. The emitter-base junction is forward-biased to inject majority carriers into the base, while the base-collector junction is reverse-biased in active operation, enabling controlled current flow from collector to emitter.

Operation: With forward bias of the emitter-base junction, a small base current Ib modulates a much larger collector current Ic, approximately Ic = β Ib, where β is the current gain. The device can operate in active, saturation, and cutoff regions; in the active region, Ic is proportional to Ib and largely independent of Vce. In saturation both junctions are forward biased; in cutoff both are reverse biased.

Electrical characteristics: Key parameters include current gain β, collector-emitter voltage Vce, and power dissipation. Silicon devices typically have Vceo around 30–1000 V and β values spanning tens to thousands, depending on grade and part.

Applications: BJTn devices are used as linear amplifiers in audio and RF circuits, as switches in digital logic and power control, and as drivers for other components. They are widely used in both analog and digital electronics, though modern designs may favor MOSFETs for some functions.

Symbol: The schematic symbol shows three terminals labeled E, B, and C, with the emitter arrow pointing outward for an NPN device.