2-21. The bias batteries in this figure have been labeled VCC for the collector voltage
supply and VBB for the base voltage supply. Also notice the base supply battery is quite
small, as indicated by the number of cells in the battery, usually 1 volt or less. However,
the collector supply is generally much higher than the base supply (normally around 6
volts). Later you will see that this difference in supply voltages is necessary in order to
have current flow from the emitter to the collector.
2-22. As stated earlier, the current flow in the external circuit is always due to the
movement of free electrons. Therefore, electrons flow from the negative terminals of the
supply batteries to the N-type emitter. This combined movement of electrons is known as
emitter current (IE). Since electrons are the majority carriers in the N-type material, they
will move through the N-type material emitter to the emitter-base junction. With this
junction forward biased, electrons continue on into the base region. Once the electrons are
in the base, which is a P-type material, they now become minority carriers. Some of the
electrons that move into the base recombine with available holes. For each electron that
recombines, another electron moves out through the base lead as base current IB (creating a
new hole for eventual combination) and returns to the base supply battery VBB. The
electrons that recombine are lost as far as the collector is concerned. Therefore, in order to
make the transistor more efficient, the base region is made very thin and lightly doped.
This reduces the opportunity for an electron to recombine with a hole and be lost. So, most
of the electrons that move into the base region come under the influence of the large
collector reverse bias.
2-23. This bias acts as forward bias for the minority carriers (electrons) in the base and,
as such, accelerates them through the base-collector junction and on into the collector
region. Since the collector is made of an N-type material, the electrons that reach the
collector again become majority current carriers. Once in the collector, the electrons move
easily through the N-type material and return to the positive terminal of the collector
supply battery VCC as collector current (IC).
2-24. To further improve on the efficiency of the transistor, the collector is made
physically larger than the base for the following reasons:
To increase the chance of collecting carriers that diffuses to the side as well as
directly across the base region.
To enable the collector to handle more heat without damage.
2-25. Total current flow in the NPN transistor is through the emitter lead. Therefore, in
terms of percentage, IE is 100 percent. On the other hand, since the base is very thin and
lightly doped, then a smaller percentage of the total current (emitter current) will flow in
the base circuit than in the collector circuit. Usually no more than 2 to 5 percent of the total
current is base current IB while the remaining 95 to 98 percent is collector current (IC). A
very basic relationship exists between these two currents: IE = IB + IC
2-26. In simpler terms, this means that the emitter current is separated into base and
collector current. Since the amount of current leaving the emitter is solely a function of the
emitter-base bias and because the collector receives most of this current, then a small
change in emitter-base bias will have a far greater affect on the magnitude of collector
current than it will have on base current. In conclusion, the relatively small emitter-base
bias controls the relatively large emitter-to-collector current.
23 June 2005