 |
Bipolar Junction Transistors
 |
A bipolar junction transistor (BJT) is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes.
Although a small part of the transistor current is due to the flow of majority carriers, most of the transistor current is due to the flow of minority carriers and so BJTs are classified as 'minority-carrier' devices.
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.
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.
A convenient mnemonic device for identifying the symbol for the NPN transistor is "not pointing in".
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. Few transistors used today are PNP, since the NPN type gives better performance in most circumstances.
PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped material. PNP transistors are commonly operated with the collector at ground and the emitter connected to a positive voltage through an electric load. A small current flowing from the base allows a much greater current to flow from the emitter to the collector.
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.
A mnemonic for identifying the symbol for the PNP transistor is "points in proudly".
Symbol of PNP Bipolar Junction Transistor.
Symbol of PNP Bipolar Junction Transistor.