Bipolar Junction Transistor (BJT)

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:

One P-N junction is between the emitter and the base; the other P-N junction is between the collector and the base. Note that the emitter and collector are usually doped somewhat differently, so they are rarely electrically interchangeable. While the terms "collector" and "emitter" go back to vacuum tube days, the base derives its name from the first point-contact transistors -- here the center connection also formed the mechanical base for the structure. In modern practice, the base region is made as thin as possible to achieve reasonable levels of current gain; it is often only about one millionth of a meter thick.

Bipolar transistors are classified as either NPN or PNP according to the arrangement of their N-type and P-type materials. Their basic construction and chemical treatment is implied by their names. So an NPN transistors is formed by introducing a thin region of P-type material between two regions of N-type material.

On the other hand, a PNP transistor is formed by introducing a thin region of N-type material between two regions of P-type material.

Since the majority and minority current carriers are different for N-type and P-type materials, it stands to reason that the internal operation of the NPN and PNP transistors will also be different. These two basic types of transistors are represented using the following symbols