Bipolar Junction Transistor Operation-Types-Applications

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Definition of Bipolar Junction Transistor

The name Bipolar came from the fact that the device operation depends on movement of charge carriers with both polarities (holes and electrons). Transistor means transfer + resistor which has the capability of transferring the signal from high resistance to low resistance or vice verse. It possesses two junctions. By combining above three statements this device is called as bipolar junction transistor.

Terminals of BJT

BJT has three terminals a) Emitter b) base c) collector analogous to cathode gate and anode in vacuum tube. Emitter is heavily doped, collector is moderately doped and base is lightly doped as we need most of the current from emitter should reach collector; base current is needed only as a pilot signal for collector current variation. Transistor is active device which has the ability to control the electron flow through it.


History of Transistor

Bipolar Junction transistor is a three terminal device commonly termed as BJT was invented in Bell laboratories in 1948 by William Shockley, Brattain, john Bardeen which revolutionized not only the world of electronics but also our day to day life. Soon after its invention it replaced vacuum tubes that were ubiquitous electronics device in all electronic circuits at that time. Quantum mechanics, physics of atomic scale particles made it possible the invention of semiconductor devices like transistor (The concepts of band, Fermi level are introduced through quantum mechanics).

Types of Bipolar junction transistors

BJT’s are of two types:

PNP transistor

PNP transistor is formed by sandwiching N-type semiconductor between two P-type semiconductors. The symbol of PNP transistor is as shown in the figure

PNP symbol

PNP symbol

PNP Transistor

PNP Transistor

The direction of arrow signifies the direction of emitter current when base emitter junction is forward biased. When base emitter junction is forward biased conventional current (flow of positive charge carriers) is from P-type emitter to N-type base. The Current flow in a PNP transistor is through majority charge carriers which are holes in PNP transistor.

NPN transistor

NPN transistor is formed by sandwiching P-type semiconductor between two N type semiconductors. The symbol of a NPN transistor is shown in the figure

NPN transistor

NPN transistor

NPN symbol

NPN symbol

The direction of arrow signifies the direction of emitter current when base emitter junction is forward biased. When base emitter junction is forward biased conventional current (flow of positive charge carriers) in to from N-type emitter through P-type base. The Current flow in a NPN transistor is through majority charge carriers which are electrons in NPN transistor.Mostly NPN transistors are used in digital logic families as electrons has less effective mass compared to holes in PNP transistor due to which speed of operation is high in NPN transistor compared to PNP transistor.


Working Principle of transistor

The basic principle involved is the use of Voltage between two terminals (commonly base and emitter terminals) to control the current flowing through third terminal(collector terminal). For example if you take the common emitter configuration as shown in figure below.

CE transistor

CE transistor

The change in VBE voltage effects the current entering in Base terminal which is called Ib, this current will inturn effect the output current called IC. By this it is illustrated that the input current(IB) controls the flow of output current(IC). So BJT is a current control device.  

Different configurations of BJT

BJT can be operated in three configurations a) Common Base b) Common emitter c) Common collector configurations. Due to the presence of two junctions each junction can be operated in forward or reverse bias leading to three different regions of operation.

Transistor as a Switch

Transistor can be used as a switch in logic gates will be operated in extreme regions of input output characteristics in which both regions will be forward biased(On state) or reverse biased(Off state) which are called saturation and cut off regions of operation simultaneously.

Transistor as an amplifier

Transistor when used as amplifier is operated in active region in which input junction will be forward biased and output junction will be reverse biased. There are three types of operating modes of amplifier i.e. Common Base (CB) amplifier, Common Emitter(CE) amplifier and Common Emitter(CE) amplifier. Other region inverse active region is of less importance in practice.

Advantages of BJT over other transistor elements

Soon after the invention of BJT new technologies emerged and MOSFET, CMOS technologies gained prominence still BJT are widely used in discrete analogy applications due to the following advantages.

  1. High driving capability.
  2. High frequency of operation.
  3. BJT’s possess high gain bandwidth product. High Gain bandwidth product of BJT allows trade off between gain and bandwidth over a wide range.
  4. Even the fastest digital logic family available Emitter coupled logic uses BJT’s as digital switches.

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