Introduction to BJT
BJT stands for Bi-polar junction transistor. In Bi-polar junction transistor the resistance of the channel is controlled by a current at the gate. Can be thought of as a current-controlled resistor. FET (Field Effect Transistor) is the other major type of transistor.
The BJT is constructed with three doped semiconductor regions separated by two pn junctions. The three regions are called emitter, base, and collector. Physical representations of the two type of BJTs are. One type consists of to n regions separated by a p region (npn) and the other type consists of two p regions separated by an n region (pnp).
The pn junction joining the base region and the emitter region is called the base-emitter junction. The pn junction joining the base region and the collector region is called the base-collector junction. A wire lead connects to each of the three regions, s shown below. These leads are labeled E,B, and C for emitter , base and collector respectively. The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions.