Bipolar Transistor

Bipolar Transistor
N-P-N Transistor
P-N-P Transistor

Bipolar Transistor



A Bipolar Junction Transistor (a.k.a. a BJT or Bipolar Transistor) is an active semiconductor device formed by two P-N junctions whose function is amplification of an electric current.

Bipolar transistors are made from 3 sections of semiconductor material (alternating P-type and N-type), with 2 resulting P-N junctions. Schematically, a bipolar transistor can be thought of in this fashion shown aside:

As PN junction diode, BJT follows:

Forward biasing and reverse biasing. Forward Biasing and Reverse biasing








The Bipolar Transistor are of two types:




N-P-N Transistors







P-N-P Transistors







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N-P-N Transistors


NPN is one of the two types of bipolar transistors, in which the letters "N" and "P" refer to the majority charge carriers inside the different regions of the transistor. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation.

NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. In other terms, an NPN transistor is "on" when its base is pulled high relative to the emitter.

The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

One mnemonic device for identifying the symbol for the NPN transistor is
"not pointing in, or 'not pointing, no' "


Current Flow in NPN transistor






















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P-N-P Transistors

The other type of BJT is the PNP with the letters "P" and "N" referring to the majority charge carriers inside the different regions of the transistor.

PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped material. A small current leaving the base in common-emitter mode is amplified in the collector output. In other terms, a PNP transistor is "on" when its base is pulled low relative to the emitter. The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

One mnemonic device for identifying the symbol for the PNP transistor is

"pointing in proudly, or 'pointing in - pah'


Current flow in PNP transistor























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