A bipolar junction transistor, or BJT, contains three doped regions: emitter, base and collector. In an npn transistor, a thin p-type base lies between n-type emitter and collector regions.

In forward-active operation, the base-emitter junction is forward biased while the base-collector junction is reverse biased. The emitter injects carriers into the thin base, and most of those carriers are swept into the collector before recombining.

A relatively small base current can therefore control a much larger collector current. A common circuit-level approximation is

$$I_C\approx \beta I_B,$$

where $\beta$ is the current gain, although its value varies substantially between devices and operating points.

The collector current is more fundamentally controlled by the base-emitter voltage, with an approximately exponential dependence in forward-active operation.

Unlike a MOSFET gate, the base-emitter junction draws current. This difference strongly affects how BJTs are biased and driven.