Junctiontransistorer

Junctiontransistorer is a three-terminal semiconductor device used to amplify or switch electronic signals. It is equivalent to a bipolar junction transistor (BJT) and consists of two PN junctions formed by three regions: emitter, base, and collector. The interfaces between the emitter-base and base-collector junctions define the transistor's main operating regions.

In active operation, the emitter-base junction is forward-biased and the base-collector junction is reverse-biased. A small current into the base controls a much larger current from the collector, with current gain beta approximately Ic/Ib. The emitter current Ie is the sum of base and collector currents (Ie = Ib + Ic).

Construction typically uses silicon or germanium, with the base lightly doped and the emitter and collector more heavily doped to maximize gain. Devices are manufactured as discrete components or integrated into circuits, and they can be designed as NPN or PNP types. Variants include vertical and lateral BJTs, and homojunction or heterojunction structures.

Applications include analog amplification, switching, and RF circuits, especially before MOSFETs became dominant. The first practical BJTs were developed in 1947 at Bell Labs. While many modern circuits rely on field-effect transistors, junctiontransistorer devices remain in use for their linearity, gain, and fast switching in certain niches.