2-14
Figure 2-10.—The reverse-biased junction in a PNP transistor.
PNP JUNCTION INTERACTION.—The interaction between the forward- and reverse-biased
junctions in a PNP transistor is very similar to that in an NPN transistor, except that in the PNP transistor,
the majority current carriers are holes. In the PNP transistor shown in figure 2-11, the positive voltage on the
emitter repels the holes toward the base. Once in the base, the holes combine with base electrons. But again,
remember that the base region is made very thin to prevent the recombination of holes with electrons.
Therefore, well over 90 percent of the holes that enter the base become attracted to the large negative
collector voltage and pass right through the base. However, for each electron and hole that combine in the
base region, another electron leaves the negative terminal of the base battery (V
BB) and enters the base as
base current (IB). At the same time an electron leaves the negative terminal of the battery, another electron
leaves the emitter as IE (creating a new hole) and enters the positive terminal of VBB. Meanwhile, in the
collector circuit, electrons from the collector battery (VCC) enter the collector as Ic and combine with the
excess holes from the base. For each hole that is neutralized in the collector by an electron, another electron
leaves the emitter and starts its way back to the positive terminal of VCC.
