Bipolar Junction Transistors (BJT) — A/L Physics

Bipolar Junction Transistor (BJT) A three-terminal semiconductor device that uses both electron and hole charge carriers to amplify or switch electrical signals. The BJT consists of two pn-junctions formed by three doped regions: emitter, base, and collector. It is called "bipolar" because both majority and minority carriers contribute to current flow. In amplifiers, a small base current controls a much larger collector current.

NPN vs. PNP Transistors NPN has a p-type base sandwiched between two n-type regions; PNP has an n-type base between two p-type regions. In NPN, electrons are the majority carriers in the emitter and collector; in PNP, holes are the majority carriers. Circuit symbols differ by the arrow direction on the emitter (pointing out for NPN, in for PNP). For Sri Lankan syllabus, NPN is more commonly used in examples.