Fig above shows a transistor operated in active region. It can be noted from the diagram the battery VEB forward biases the EB junction while the battery VCB reverse biases the CB junction.
As the EB junction is forward biased the holes from emitter region flow towards the base causing a hole current IPE. At the same time, the electrons from base region flow towards the emitter causing an electron current INE. Sum of these two currents constitute an emitter current IE = IPE +INE.
The ratio of hole current IPE to electron current INE is directly proportional to the ratio of the conductivity of the p-type material to that of n-type material. Since, emitter is highly doped when compared to base; the emitter current consists almost entirely of holes.
Not all the holes, crossing EB junction reach the CB junction because some of the them combine with the electrons in the n-type base. If IPC is the hole current at (Jc) CB junction. There will be a recombination current IPE - IPC leaving the base as shown in figure.
If emitter is open circuited, no charge carriers are injected from emitter into the base and hence emitter current IE =o. Under this condition CB junction acts a a reverse biased diode and therefore the collector current ( IC = ICO) will be equal to te reverse saturation current. Therefore when EB junction is forward biased and collector base junction is reverse biased the total collector current IC = IPC +ICO.