April 27, 2011

Bipolar Junction Transistor ( BJT )

Bipolar transistors are semiconductor devices consist of three electrodes; the emitter, base and collector. Like diodes, bipolar transistors can be constructed of silicon or germanium but silicon is the most popular. Bipolar transistors are commonly used as amplifier and electronic switch. There are two types of bipolar transistor, NPN and PNP.
NPN transistor is formed by sandwiching a P-type material between two N-type materials. The other type is PNP transistor which is formed by sandwiching an N-type material between two P-type materials. In both types of bipolar transistors, the central region is called the base while the outer regions are called collector and emitter.

Schematic and Block Diagram of Bipolar Junction Transistor - Click to enlarge


There are rules to be followed in biasing transistor. One of the rules is that, the emitter-base junction should always be forward biased. This means that P-type material should be more positive and N-type material should be more negative. For a silicon transistor, there must be at least 0.7VDC bias across emitter-base junction in order for bipolar transistor to turn on. For germanium, it must be at least 0.3VDC. The other rule is that, the collector-base junction should be reverse-biased, which means that the N-type material should be more positive compared to the P-type material.

Bipolar Junction Transistor Biasing - Click to enlarge

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