In Fig. 3.5 both biasing potentials have been applied to a pnp transistor, with the
resulting majority- and minority-carrier flow indicated. Note in Fig. 3.5 the widths of
the depletion regions, indicating clearly which junction is forward-biased and which
is reverse-biased. As indicated in Fig. 3.5, a large number of majority carriers will
diffuse across the forward-biased p-n junction into the n-type material. The question
then is whether these carriers will contribute directly to the base current IB or pass
directly into the p-type material. Since the sandwiched n-type material is very thin
and has a low conductivity, a very small number of these carriers will take this path
of high resistance to the base terminal. The magnitude of the base current is typically
on the order of microamperes as compared to milliamperes for the emitter and collector
currents. The larger number of these majority carriers will diffuse across the
reverse-biased junction into the p-type material connected to the collector terminal as
indicated in Fig. 3.5. The reason for the relative ease with which the majority carriers
can cross the reverse-biased junction is easily understood if we consider that for
the reverse-biased diode the injected majority carriers will appear as minority carriers
in the n-type material. In other words, there has been an injection of minority carriers
into the n-type base region material. Combining this with the fact that all the
minority carriers in the depletion region will cross the reverse-biased junction of a
diode accounts for the flow indicated in Fig. 3.5.