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A bipolar transistor (bipolar junction transistor: BJT) consists of the collector, base, and emitter regions, with the very thin base region located between the collector and emitter regions. The base region shares two pn junctions, each with collector and emitter. To obtain a high current gain, the emitter region is orders of magnitude more heavily doped than the base region. In this way, a bipolar transistor is formed by two back-to-back diodes.
When each terminal is at the specified voltage, the collector draws a current that is hFE times higher than the current applied to the base.
Let’s consider the npn transistor where the collector potential is higher than the emitter potential and the base potential is roughly 0.7 V higher than the emitter potential. In other words, the base-emitter junction is forward-biased whereas the base-collector junction is reverse-biased.
When the base-emitter junction is forward-biased, a small current flows into the base, injecting holes into the p-doped base region. As a result, these holes attract electrons from the emitter into the base region across the forward-biased base-emitter junction. Since the emitter region is very heavily doped, many more electrons enter the base region than holes, and some of the electrons recombine with holes. Most of the remaining electrons are swept across the very thin base region and contribute to the collector current.
Next, let’s consider the pnp transistor. Suppose that the collector potential is lower than the emitter potential and that the base potential is roughly 0.7 V lower than the emitter potential. In the case of the pnp transistor, electrons are injected into the n-doped base region. Therefore, holes are attracted from the emitter into the base region. Some of these holes recombine with the electrons injected into the base region. The remaining holes diffuse across the base region, reaching the collector.