Bipolar junction transistors, electronic components, transistor, transistors, bipolar junction transistor

A bipolar junction transistor (BJT) is a three-terminal semiconductor device that can amplify signals or act as an electronic switch. It is formed from three regions of doped semiconductor arranged as either NPN or PNP, giving it two junctions. The three terminals are the base, the collector and the emitter. A small current flowing into the base controls a much larger current flowing between the collector and the emitter, and this current-controlling action is what makes the transistor so useful.

In an NPN transistor a small base current allows a larger collector current to flow, and the ratio between them is the device’s current gain. Used as an amplifier, the transistor turns a small varying input current into a larger copy at the output, the basis of audio and radio-frequency amplification. Used as a switch, it is driven either fully on or fully off, so that a small control signal can turn a much larger load on and off, as in logic circuits and relay drivers.

The behaviour of a bipolar transistor depends on the small forward voltage needed to turn on its base-emitter junction and on its current gain, both of which vary with the device and with temperature. Bipolar transistors are valued for their ability to handle current and for good high-frequency performance. They contrast with field-effect transistors, which are controlled by a voltage rather than a current; both types are fundamental building blocks of modern electronic circuits.

Frequently asked questions

What are the three terminals of a bipolar transistor?: They are the base, the collector and the emitter. A small current into the base controls the much larger current that flows between the collector and the emitter.
What is the difference between NPN and PNP?: They are the two arrangements of the transistor’s semiconductor regions. They operate with opposite polarities of voltage and current, but both control a large current with a small base current.
How does a transistor work as a switch?: The transistor is driven either fully off, blocking current, or fully on, conducting freely. This lets a small control signal turn a much larger load on and off.

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Bipolar junction transistors a bipolar transistor uses a small current to control a larger current, a little like a relay. Bipolar transistors generally have 3 terminals. The control terminal is called the base. The other 2 terminals are known as the emitter and the collector and they carry virtually all of the current flowing through the transistor

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