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The information presented in this cross reference is based on TOSHIBA's selection criteria and should be treated as a suggestion only. Please carefully review the latest versions of all relevant information on the TOSHIBA products, including without limitation data sheets and validate all operating parameters of the TOSHIBA products to ensure that the suggested TOSHIBA products are truly compatible with your design and application.
Please note that this cross reference is based on TOSHIBA's estimate of compatibility with other manufacturers' products, based on other manufacturers' published data, at the time the data was collected.
TOSHIBA is not responsible for any incorrect or incomplete information. Information is subject to change at any time without notice.

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What is a bipolar transistor?

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.

What is a bipolar transistor?
Operating regions of a bipolar transistor
Operating regions of a bipolar transistor

Chapter 3, “Transistors,” of the e-learning session provides related information.

For the operation of a bipolar transistor, see the FAQ entry, “How do npn and pnp transistors operate?”