2-30. 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.
The difference is that in the PNP transistor, the majority current carriers are holes. In the
PNP transistor (see Figure 2-13), the positive voltage on the emitter repels the holes toward
the base. Once in the base, the holes combine with base electrons. 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
combines in the base region, another electron leaves the negative terminal of the base
battery (VBB) 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.
2-31. Although current flow in the external circuit of the PNP transistor is opposite in
direction to that of the NPN transistor, the majority carriers always flow from the emitter to
the collector. This flow of majority carriers also results in the formation of two individual
current loops within each transistor. One loop is the base-current path and the other loop
the collector-current path. The combination of the current in both of these loops (IB + IC)
results in total transistor current (IE). The most important thing to remember about the two
different types of transistors is that the emitter-base voltage of the PNP transistor has the
same controlling affect on collector current as that of the NPN transistor. In simple terms,
increasing the forward-bias voltage of a transistor reduces the emitter-base junction barrier.
This action allows more carriers to reach the collector causing an increase in current flow
from the emitter to the collector and through the external circuit. A decrease in the
forward-bias voltage reduces collector current.
Figure 2-13. PNP Junction Interaction Transistor Operation
23 June 2005