The fundamental theory behind a bipolar transistor comes in its layered design.  In the case of an NPN Transistor this includes an N region (electron majority carriers) a P region (hole majority carriers) and another N region.  PNP Transistors contain two P-Type layers with a central N-Type layer. These layers form three configuration leads: a base, a collector, and an emitter.  In the active mode, the Emitter/Base junction (EBJ) is forward biased, while the Collector/Base junction (CBJ) is reverse biased.  In saturation mode, both the EBJ and CBJ are forward biased while in cutoff, both junctions are reverse biased.  The remaining configuration is called ‘Reverse Active’, in which the EBJ is reverse biased and the CBJ is forward biased.

In each of these configurations, special characteristics and functions can be exploited, making the BJT transistor a very functional and fundamental device in non-linear circuit design.

What Are BJTs Used For...

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