Operations Of Transistors

OPERATION OF NPN TRANSISTOR:

• The forward bias is applied to the emitter base junction of an NPN transistor,    causes  a  lot  of  electrons  from  the  emitter  region  to  cross  over  to  the  base region.

• As the base is lightly doped with P type impurity, the number of holes in the base  region  is  very  small  and  hence  the  number  of  electrons  that  combines with the holes in the P type region is very small.

• Hence, a few electrons combine with the holes to constitute base current IB.    the  remaining  electrons  crossover  into  the  collector  region  to  constitute  a collector current IC. Thus the base and collector current summed up gives the emitter current IE = IC + IB.

OPERATION OF PNP TRANSISTOR

• The  forward  bias  applied  to  the  emitter  base  junction  of  a  PNP  transistor causes a lot of holes from the emitter region to cross over to the base region as the base is lightly doped with N type impurity.

• The  number  of  electrons  in  the  base  region  is  very  small  and  hence  the number  of  holes  combined  with  electrons  in  the  N  type  base  region  is  also very small.

• Hence  a  few  holes  combined  with  electrons  to  constitute  a  base current  IB. The  remaining  holes  cross  over  into  the  collector  region  to  constitute  a collector current IC. Thus IE = IB + IC.

• Besides hole current, there is electron current which flows from base region to emitter  region.  This  current  depends  upon  emitter  base  potential.  As  the width of the base region is very small, the ration of hole current to electron current is very small. Thus the electron current may be neglected.