Bipolar Junction Transistor

Bipolar junction transistor is a special type of transistor that consists of three terminals connected to three doped semiconductor regions separated by two p-n junctions. This transistor is commonly known as Bipolar transistor or BJT.

Bipolar junction transistor (BJT)

Bipolar transistors are manufactured in two types, PNP and NPN, and are available as separate components, usually in large quantities. The prime use or function of this type of transistor is to amplify current. This makes them useful as switches or amplifiers. They have a wide application in electronic devices like mobile phones, televisions, radio transmitters and industrial control.

Function of Bipolar Junction Transistor

BJTs are of two types namely NPN and PNP based on doping types of the three main terminals. An NPN transistor consists of two semiconductor junctions that have a thin p-doped anode region and PNP transistor also consists of two semiconductor junctions that have a thin n- doped cathode region.

npn and pnp transistor

The flow of charge in a Bipolar transistor is due to diffusion of charge carriers between two regions belong to different charge concentrations. Regions of BJT are known as the base, collector, and emitter.

The emitter region is highly doped when compared to other layers. Both collector and base layers have the same charge carrier concentrations. Among these junctions, the base-emitter junction is forward biased, and base-collector junction is reverse biased. Forward biased means p-doped has it contains more potential than the n-doped side.

Voltage, Charge Control and Current

The base-emitter current is controlled by collector-emitter current. This conclusion is drawn by the current-voltage relation of the base-emitter junction. Collector current has a base region where minority carriers are concentrated.

Transistor models such as Glenn poon model is responsible for the distribution of the charge which explains the behaviour of a transistor.

Bipolar Transistors Configurations

Since BJT is a three terminal device, there are three ways to connect it within an electric circuit while one terminal is same for both output and input. Every method of connection responds differently to the input signals within a circuit.

  • Common Emitter Configuration – has both voltage and current gain
  • The common Collector Configuration – has no voltage gain but has current gain
  • The common base configuration – has no current gain but has voltage gain

Characteristics of different transistor configurations are given in the following table:

Characteristics Common Base Common Emitter Common Collector
Power Gain low Very high medium
Current gain low medium high
Voltage gain High Medium low
Phase angle 0 180 0
Output impedance Very high high low
Input Impedance Low medium high

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