Let us consider a npn transistor
with emitter-base
junction forward biased and collector-base junction reverse-biased as shown.
Lets understand the working of a transistor from
the figure.
The -ve terminal of the battery
VEB repels electrons
of the emitter and since emitter-base junction is forward biased, the electrons
cross over the junction easily to reach base. As the base is very thin and
lightly doped,
therefore a very few electrons, less than 5% , from the emitter
combine with the holes of the base and the remaining electrons are pulled by
the collector.
The collector reached electrons go through the
batteries VCB and VEB to finally reach back the emitter. These
electrons from collector passing through
the battery VCB constitute
the collector current Ic . This is the story of more than 95% electrons coming from emitter.
The remaining less than 5% electrons which do not get
a chance to reach collector, due to light doping and thin width of the base, combine
with holes of the base.
As soon as an electron combines with a hole in the
base, an electron and a hole is created in the base. The created electron
reaches +ve terminal of battery VEB thus constituting base current IB . Simultaneously, -ve terminal of VEB sends an electron to emitter.
Thus, emitter
current = base current + collector current
IE = IB + IC
and IB
<< IC
So, this was all about working of BJT .
