Introduction to Bipolar Transistors BJT !

Introduction to Bipolar Transistors BJT !
Introduction to Bipolar Transistors BJT !
Introduction to Bipolar Transistors BJT !

Since their introduction shortly after the Second World War, transistors have revolutionised the world of electronics. Their ever decreasing size and their versatility have made them into perhaps the most useful and important electronic components in existence. In this course we will be covering two of the possible configurations for basic transistor circuits.

Introduction to Bipolar Transistors BJT !In the original transistors, known as bipolar transistors, two semiconductor layers surround a third layer.

Bipolar transistors have semiconductor material of the same conductivity type on each of the two sides. Located in between is a thin layer of the opposite type. The two sides are designated the collector and emitter zones; the layer in the middle is termed the base zone. At a first glance, this arrangement looks like two diodes linked together. You might expect that no current is able to flow between the collector and the emitter, given that a reverse voltage is always being applied to one of the two "diode junctions". The secret to this arrangement lies in the thinness of the base zone. Charge carriers are always able to bridge a small distance extending beyond the junction. If the base zone is supplied with additional charge carriers by applying a voltage to it, sufficient charge carriers are present at any particular point to, as it were, bridge over the gap, and current begins to flow. Because of the fact that the current flow between the collector and the emitter is only "switched on" when a sufficient voltage and current supply is present at the base, transistors can be used as electronic switches which can be switched on and off depending on the current fed to the base.

This process is illustrated by the following example of an NPN transistor.

Introduction to Bipolar Transistors BJT !

The configuration of the layers and the circuit symbol for each of the two different types of bipolar transistor is shown in the following diagrams.

Introduction to Bipolar Transistors BJT !

Just how much current flows between the collector and the emitter depends on the number of charge carriers in the base zone, which means that changes in the voltage and amperage at the base can lead to a stronger or weaker current flow between the collector and emitter. Even a slight change at the base results in a substantial change to the voltage between the collector and the emitter. This relationship remains linear over a wide range: a changed voltage signal at the base is reproduced exactly between the collector and the emitter, but with a considerably greater voltage change. The transistor therefore amplifies the signal. This is the second standard area of application for transistors.
 

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