Bipolar Junction Transistors

2. Action of transistor

  • Transistor has two junctions
    1. Emiiter base (EB) junction
    2. collector base(CB) junction
  • To operate the transistor a suitable potential difference must be applied across two of its junction .This is known as biasing of the transistor
  • The charge carriers move across different region of the transistor whne all three terminals of transistors are properly biased
  • There are four possible ways to bias these functions given belwo in the table

    Condition EB junction CB junction Region of operation
    1 FB RB Active
    2 FB FB Saturation
    3 RB RB cut off
    4 RB FB Inverted

  • Only one of these conditions interests us at the moment that is condition 1 in which EB junction is FB and CB junction is RB
  • We will consider the case of npn transistor when its EB junction is FB and CB junction is RB.Case of pnp transistor can be also be understood on the same basis
  • the figure 2 shows that an npn transistor is properly biased using the two power supplies VEE and VCC and base terminal is common to both these power supplies

  • Consider figure 2a ,since emmiter base terminal is FB the majority charge carriers there are electrons in this case enters the base region in large numbers
  • Base region which is very lightly doped is short of majority charge carriers which are holes in case of npn transistors.Since base is a p-type semiconductor ,thus large numbers of electrons entering the base from the emitter combines with the very small number of holes present there in base region
  • Since the base collector junction is RB so these electrons which are minorty charge carriers in base ,can easily cross the jucntion and enter the collectror region
  • These electron just entered the collector region moves forwards to reach the collector terminal
  • These electron go through batteries Vcc and VEE and are taken back to emmiter
  • The electrons going from source VEE to emitter constitute the electric current IE in the direction oppostie to the direction of the flow of electron .This current is known as emmitter current.

  • Similary electron moving from the collector to the battery constitute the collectror current IC
  • Collector current IC is slightly less then that of emitter current IE becuase some of the electrons comes out of the base terminal instead of going to the collector
  • This small fraction of current comming out of base terminal is known as base current IB.Base current is a small fraction of the emiiter current
  • Using kirchoof law we can write
    IE=IC+IB          ---(1)
  • There is always a reverse leakage current in the transistor whioninorty charge carriers
  • α and β parameters for transistor are defined as
    β =IC/IB           ----(2)
  • These parameters are helpful while analyzing transistors as a circuit element
  • We have to keep one connection in mind that all currents entering into transistor are taken to be positive and current flowing out is negative
  • Thus for an NPN transistor
    IE -> negative
    IC ,IB-> positive

Class 12 Maths Class 12 Physics