Baseemitter

Base-emitter refers to the junction between the base and emitter regions of a bipolar junction transistor (BJT). In a typical NPN transistor, the emitter is heavily doped N-type, the base is P-type, and the collector is N-type. The base-emitter junction acts as a diode; when forward biased, it injects carriers from the emitter into the base and enables transistor action. The base region is thin and lightly doped to allow most injected carriers to diffuse into the collector region.

In normal operation, the base-emitter junction is forward biased with a voltage around 0.6 to 0.7 volts for silicon devices. This forward bias creates a large emitter current that, through the transistor’s action, modulates a much larger collector current. In active region, the base-collector junction is reverse biased, which helps to direct carriers from the emitter to the collector. If both junctions are forward biased, the device enters saturation.

Electrical relationships are commonly described by Ic = β Ib in active mode, where Ic is the collector current, Ib is the base current, and β is the current gain. The emitter current is Ie = Ib + Ic ≈ (β+1)Ib. The base-emitter voltage is temperature dependent; it typically decreases by about 2 mV per degree Celsius, and the diode-like behavior of the BE junction is often described by an exponential I–V relationship at a given temperature.

The base-emitter junction is central to biasing strategies, amplification, and switching in analog and mixed-signal circuits, and is commonly used in diode-connected configurations for temperature compensation and references.