Structure and Operation of the Bipolar Transistor

Structure of the Bipolar Transistor

The bipolar transistor (BJT) is a component made of three doped semiconductor layers, forming two back-to-back PN junctions: NPN or PNP type. The three terminals are the emitter (E), the base (B), and the collector (C). The base is very thin and lightly doped, which is essential to the transistor effect.

Transistor Effect

In active mode, the base-emitter junction is forward-biased and the base-collector junction is reverse-biased. The majority carriers injected by the emitter cross the thin base without recombining and are collected by the collector. The collector current Ic is then proportional to the base current Ib:

Ic = beta * Ib

where beta (also denoted hFE) is the static current gain, typically between 50 and 300.

The Three Operating Modes

Mode	BE Junction	BC Junction	Use
Cutoff	reverse	reverse	open switch
Saturated	forward	forward	closed switch
Active	forward	reverse	amplification

Common Pitfall

Confusing the saturated mode with the active mode: in saturation, the relation Ic = beta*Ib no longer applies, and Vce drops to a nearly zero value (Vce_sat ~ 0.2 V). To amplify a signal without distorting it, the transistor must remain in active mode.

Example

For an NPN transistor with beta = 100 and Ib = 20 microA, we get Ic = 100 * 20e-6 = 2 mA, as long as the transistor is not saturated (Vce remains above Vce_sat).