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4. BJT Fundamentals > 4-3 Transistor Mechanism

4-3 TRANSISTOR MECHANISM

The basic operation of the transistor is described using the p–n–p transistor. The p–n junction of the transistor is forward-biased whereas the base-to-collector is without a bias, as shown in the Fig. 4-2.

Figure 4-2 Forward-biased junction of a p–n–p transistor

Figure 4-2 Forward-biased junction of a p–n–p transistor

The middle portion is termed as the base (B) while the two end portions are known as the emitter (E) and the collector (C). The junction between the emitter and the base is called the emitter–base junction, or the emitter junction (JE). The junction between the collector and the base is called the collector-base junction, or briefly the collector junction (JC).

The depletion region gets reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from the p-type to the n-type material gushing down the depletion region and reaching the base. The forward-bias on the emitter–base junction will cause current to flow. This flow of current consists of two components: (i) holes injected from emitter to base and (ii) electrons injected from base to emitter. The design of the transistor ensures that the base region is fabricated very lightly compared to the emitter or the collector regions. Consequently, we have the holes injected from the emitter into the base in large numbers, and we neglect the injection of electrons from the base region.


  

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