Current across the base-emitter junction:
The solid arrows, which are labelled with subscripted I, represent current flow. For holes, motion and current flow are in the same direction, whereas for electrons, current flow and carrier motion are in opposite directions. The electrons, that are injected into the base, diffuse away through the emitter-base junction towards the (reverse biased) base-collector junction. Since they move through the base, some electrons encounter holes and recombine with them. Those electrons which get to the base-collector junction run in a large electric field that sweeps them out of the base and into the collector. (They "fall" down large potential drop at the junction.) These effects are all illustrated in Figure with arrows representing the several currents, which are linked with each of the carrier's fluxes. IEe represents the current linked with the electron injection into the base. (This points in the opposite direction through the motion of the electrons, as electrons have a negative charge.) IEh represents the current related with holes injection into the emitter from the base. IBr represents recombination current in the base, while ICe represents the electron current going into the collector. It must be easy to prove that:
I E = I Ee + I Eh
I B = I Eh + I Br
IC = ICe
In a good transistor, almost all current across the base-emitter junction consists of electrons being injected into the base. The transistor engineer works tough to design the device so that very little emitter current is built up of holes coming from the base in the emitter. The transistor is also designed so that about all electrons that are injected into the base make it across to the base-collector reverse-biased junction. Some of the recombination is unavoidable, however things are arranged so as to minimize this effect.
It shall be good to define some current gains in the transistor. First is the common emitter current achieved represented by β and it is the ratio of IC and IB (IC/IB) and its value is in the range 50-300. From the transistor model illustrated in Figure,
IE = IC + IB = (1 + β) IB
For collector and emitter currents, IC= (β/ β+ 1 ) I E = α I E and α is close unity.