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bipolar transistor

See also field-effect transistor (field-effect transistor) and transistor.

A bipolar transistor is a semiconductor device commonly used for amplification. The device can amplify analog or digital signals. It can also switch DC or function as an oscillator. Physically, a bipolar transistor amplifies current, but it can be connected in circuits designed to amplify voltage or power.

There are two major types of bipolar transistor, called PNP and NPN. A PNP transistor has a layer of N-type semiconductor between two layers of P-type material. An NPN transistor has a layer of P-type material between two layers of N-type material. In P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. In N-type material, the charge carriers are primarily electrons.

The bipolar transistor has advantages and disadvantages relative to the field-effect transistor (field-effect transistor). Bipolar devices can switch signals at high speeds, and can be manufactured to handle large currents so that they can serve as high-power amplifiers in audio equipment and in wireless transmitters. Bipolar devices are not especially effective for weak-signal amplification, or for applications requiring high circuit impedance.

Bipolar transistors are fabricated onto silicon integrated circuit (IC) chip. A single IC can contain many thousands of bipolar transistors, along with other components such as resistors, capacitors, and diodes.

This was last updated in April 2005

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When does a bipolar transistor act as a closed switch?
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When a small current is applied at the base (pulse), it acts as a closed switch. if the pulse is not there it acts as an open switch with some negligible leakage
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