Tuesday, 9 April 2013

NPN and PNP Transistor Structure

The BJT (Bipolar junction Transistor) can be constructed by using three doped semiconductor regions separated by two p-n junctions. These three regions are called as emitter, base, and collector. The emitter will emits the electrons, while the collector will collects the electrons emitted from the emitter. The emitter region and the collector regions are separated by the help of a less doped region called as the base. Based on the combination of p and n regions, there are two types of BJTs, either pnp (two p regions separated by one n region) and npn (two n regions separated by one p region).

The npn and pnp BJTs are represented below:


The easiest method to identify a transistor is just check the arrow mark represented in the transistor circuit. The arrow mark points to the n-region of the transistor. The regions near to the arrow mark show the p-region. In figure (a), the emitter section shows the arrow mark. So the emitter section is a n-region. The region near to emitter from figure is base. So from the configuration of transistor, it is clear that no two n regions occur near. Thus the Base region is P type. Also by the configuration of transistor it is clear that the side near to p-region should be n-type. Hence the transistor is a npn transistor. In the similar way from figure (b), it is clear that the arrow mark points to the base region. So the base region is n-type. The regions opposite to base should be p-regions. Hence the structure is a pnp transistor.

The C, E, and B symbols represent the collector, emitter, and base regions, respectively. The base region is lightly doped and it is very thin region. The emitter side is heavily doped and the collector region is moderately doped.


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