3.8 Summary

A bipolar junction transistor may be thought of as an extension of a simple diode or PN junction. Another layer of doped material is added, resulting in either an NPN or PNP configuration, both with two depletion regions. The two depletion regions create two hills in the energy diagram. The three terminals of the device are called the emitter, base (middle) and collector. BJTs are not normally constructed symmetrically and swapping the collector and emitter can result in unpredictable behavior.

For proper operation, the base-emitter junction is forward-biased while the collector-base junction is reverse-biased. This results in the emitter and collector currents being very nearly equal and much, much larger than the base current. The ratio of collector current to base current is called β (beta) while the ratio of collector current to emitter current is called α (alpha). β in particular is subject to wide variations and it can have a major impact on circuit parameters. A plot of collector current versus collector-emitter voltage reveals the three main regions of the BJT circuit: saturation, constant current and breakdown.

The Ebers-Moll model consists of a diode from the base to emitter and a controlled current source from the collector to base. This simple model of the BJT can be used to solve a variety of transistor circuits, particularly when used in conjunction with a DC load line. The DC load line is a plot of all possible operating points for a given transistor circuit.

Finally, it is possible to create switching and driver circuits using BJTs that produce stable output currents. These may utilize saturating or non-saturating configurations with NPN or PNP devices.