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Bipolar Junction Transistors




1. Introduction

  • Transistors were invented by J.Bardeen and W.H.Brattain of bell laboratories in USA in 1947 and that was a point contact transistor.
  • William shockey first invented junction transistor in 1951 which consists of two back to back pn junctions.
  • These transistors are also known as bipolar junction transistor (BJI) and are our subject of study in this section.
  • Thus a jucntion transistor is formed by sandwiching a thin layet of p-type semi-conductor between two layers of n-type semi-conductor and vice versa.
  • Transistors are of two types
    1. PNP transistors : Here the n-type thin layer is sandwiched between two p-type layes
    2. NPN transistors : Here p-type thin layer is sandwiched between two n-layers
  • There are three regions in a transistors
    1. emiiter
    2. base
    3. collector
  • Figure (1) shows digramatic representation of PNP and NPN transistors along with the sysmbols used to represent them



  • All the three regions of the transistors have different thickness and their doping levels are also different.
  • In symoblic representation of transistors the arrow head shows the direction of conventional current in the transistor.
  • Brief discription of all the three region of the transistors
    1. Emmitter: Emitter region of transistor is of moderate size and it is heavilly doped .Function of emiiter is to inject electrons or holes depedning on types of transitor into the base
    2. Base: Middle region of transistor is known as base region.This region of transistor is very thin and very lightly doped.Function of the base is to pass most the electron or holes onto the collector
    3. Collector: Collector region is moderately doped and is made physically larger than the emiiter since it is required to dissipate more heat .Collector has a job of collecting electrons or holes from the base.




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