Bipolar transistors are a type of transistor composed of pn junctions, which are also called bipolar junction transistors (BJTs). Whereas a field-effect transistor is a unipolar device, a bipolar transistor is so named because its operation involves two kinds of charge carriers, holes and electrons.
Since the bipolar transistor was the first transistor to be invented, when one simply says "transistors," it sometimes means bipolar transistors.
Two types of bipolar transistor are available, known as npn and pnp, based on the type of junction. The structure of a bipolar transistor looks symmetrical. (For example, in the case of an npn transistor, the collector and the emitter on both sides of the p region of the base are n regions, which look the same.) However, the dopant concentrations in the collector and emitter regions are quite different. Therefore, if the emitter and collector terminals are reversed, a bipolar transistor has a much lower hFE and does not function as intended.
When a bipolar transistor is in the active region, the collector current is basically hFE times the base current. Therefore, an amplifier circuit can be configured using the active area.
In contrast, the saturation and cut-off regions allow bipolar transistors to be used as switches because there is little electrical resistance between emitter and collector in the saturation region and little current flows in the cut-off region.