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.