by For normal operation, By applying the power in such a way that the emitter-base junction is always forward biased as its resistance is very small. The collector-base junction is reversed biased and its resistance is a bit high. A small forward biased is sufficient to apply for emitter junction whereas a large reverse-biased has to be applied to collector junction.
The direction of current shown below is the conventional current direction which is opposite to the direction of an electron and in direction of movement of holes.
The forward bias at the emitter-base junction reduces its barrier potential and as a result, electrons start to flow from the emitter into the base similarly holes also flow from the P-type base into the N-type region. Due to the lightly doped base region more electrons accumulated from the emitter into a base region which could not recombine with holes of base region. These electrons behave like minority charge carriers for reverse-biased collector base junction. Hence small reverse current flow due to these charge carriers. The more reverse-biased Collector base junction causes the collector base depletion layer to penetrate deeper into the base region. Thus base becomes thinner due to which the electron emitted from the emitter region into the base region almost immediately reaches the collector-base junction where a larger positive collector voltage is present, causing almost all of these electrons to cross the collector-base junction and flow into the collector region as collector current. Due to this phenomenon, most of the emitter current becomes a collector and very few electrons recombine with holes to give rise to base current.
The base current IB is some of the current IB1 due to the holes diffused from the base region to the emitter region and the current IB2 due to the recombination of electrons and holes in the base region.
i.e IB = IB1 + IB2
Now from the above figure, we can write:
IE = IB + IC
or 100% of IE = 1% of IB + 99% of IC
