Bipolartransistor

Bipolartransistor, commonly known as a bipolar junction transistor (BJT), is a three-terminal semiconductor device that uses both electrons and holes as charge carriers to amplify or switch electrical signals. It consists of two p–n junctions forming emitter, base, and collector regions. The emitter-base junction is typically forward-biased, while the base-collector junction is reverse-biased in the active region, allowing controlled current flow from emitter to collector.

In an NPN transistor, electrons are injected from the n-type emitter into the p-type base; in a PNP transistor, holes are injected from the p-type emitter into the n-type base. The base is thin and lightly doped so that most carriers injected by the emitter cross into the collector, producing a large current gain. The collector current is approximately β times the base current (Ic ≈ βIb), with β depending on device geometry and temperature.

Common transistor configurations include common-emitter, common-base, and common-collector (emitter follower). The device operates in regions known as active, saturation, and cutoff, with the active region used for amplification. Key electrical characteristics include current gain β and breakdown voltages such as BV_CEO and BV_EBO; leakage currents like I_CBO increase with temperature. The Early effect causes a modulation of collector current by the collector-emitter voltage.

Applications include voltage and current amplification in analog circuits, switching in digital logic, and power regulation in power electronics. Bipolar transistors are widely used in discrete form and in integrated circuits, though they compete with MOSFETs for many switching and amplification tasks.