#BIPOLAR JUNCTION TRANSISTOR FREE#
When the electrons that have recombined with holes leave the crystalline structure of the base, they become free electrons in the metallic base lead and produce the external base current.Īs the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage. The base has a low density of holes, which are the majority carriers.Ī small percentage of the total number of free electrons injected into the base region recombine with holes and move as valence electrons through the base region and into the emitter region as hole current.įigure 4: BJT operation showing electron flow These free electrons easily diffuse through the forward-based BE junction into the lightly doped and very thin p-type base region. The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons.
The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions.įigure 2 shows the schematic symbols for the npn and pnp bipolar junction transistors. The leads are labeled E, B, and C for emitter, base, and collector, respectively. The pn junction joining the base region and the collector region is called the base-collector junction.Ī wire lead connects to each of the three regions. The pn junction joining the base region and the emitter region is called the base-emitter junction. The term bipolar refers to the use of both holes and electrons as current carriers in the transistor structure.
One type consists of two n regions separated by a p region (npn), and the other type consists of two p regions separated by an n region (pnp). The BJT is constructed with three doped semiconductor regions ( emitter, base, and collector) separated by two pn junctions.
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