BJT: Summary of hole and elecfron flow in a transistor
The bipolar transistor is basically a simple device, and this section is devoted to a simple and largely qualitative view of BJT operation.
It is clear that current lE flows into the emitter of a properly biased p-n-p transistor and that lc flows out at the collector, since the direction of hole flow is from emitter to collector. However, the base current IB requiresa bit more thought. In a good transistor the base current will be very small. since IE is essentially hole current, and the collected hole current Ic is almost equal to IC. There must be some base current, however, due to requirements of electron flow into the n-type base region.
We can account for lB physically by three dominant mechanisms:
(a) There must be some recombination of injected holes with electrons in the base, even with Wb < Lp.The electrons lost to recombination must be resupplied through the base contact.
(b) Some electrons will be injected from n to p in the forward-biased emitter junction, even if the emitter is heavily doped compared to the base. These electrons must also be supplied by IB.
(c) Some electrons are swept into the base at the reverse-biased collector junction due to thermal generation in the collector. This small current reduces IB by supplying electrons to the base.
The dominant sources of base current are
(a) recombination in the base and
(b) injection into the emitter region.
Both of these effects can be greatly reduced by device design. In a well-designed transistor, IB will be a very small fraction (perhaps one-hundredth) of IE.
In an n-p-n transistor the three current directions are reversed, since electrons flow from emitter to collector and holes must be supplied to the base. The physical mechanisms for operation of the n-p-n can be understood simply by reversing the roles of electrons and holes in the p-n-p discussion.