ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 2
(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. Thus, most of
the electrons that move into the base region come under the
influence of the large collector reverse bias. This bias acts
as forward bias for the minority carriers (electrons) in the
base and, as such, accelerates them through the basecollector
junction and on into the collector region. Since the collector
is made of an Ntype material, the electrons that reach the
collector again become majority current carriers. Once in the
collector, the electrons move easily through the N material and
return to the positive terminal of the collector supply battery
VCC as collector current (IC).
To further improve the efficiency of the transistor, the
collector is made physically larger than the base for two
reasons:
(a) To increase the chance of collecting carriers that
diffuse to the side, as well as directly across the base region,
and
(b) To enable the collector to handle more heat without
damage.
In summary, total current flow in the NPN transistor is through
the emitter lead. Therefore, in terms of percentage, 1E is 100
percent. On the other hand, since the base is very thin and
lightly doped, 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
In simple 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 emitterbase
bias, and because the collector receives most of this current,
then a small change in emitterbase bias
Previous Page
