TC 9-62
PNP Transistor Operation
2-27. The PNP transistor works essentially the same as the NPN transistor. However,
since the emitter, base, and collector in the PNP transistor are made of materials that are
different from those used in the NPN transistor, different current carriers flow in the PNP
unit. The majority current carriers in the PNP transistor are holes. This is in contrast to the
NPN transistor where the majority current carriers are electrons. In order to support this
different type of current (hole flow), the bias batteries are reversed for the PNP transistor.
Figure 2-10 shows a typical bias setup for the PNP transistor. Notice that the procedure
used earlier to properly bias the NPN transistor also applies here to the PNP transistor. The
first letter (P) in the PNP sequence indicates the polarity of the voltage required for the
emitter (positive) and the second letter (N) indicates the polarity of the base voltage
(negative). Since the base-collector junction is always reverse biased, then the opposite
polarity voltage (negative} must be used for the collector. Therefore, the base of the PNP
transistor must be negative with respect to the emitter and the collector must be more
negative than the base. Just as in the case of the NPN transistor, this difference in supply
voltage is necessary in order to have current flow (hole flow in the case of the PNP
transistor) from the emitter to the collector. Although hole flow is the predominant type of
current flow in the PNP transistor, hole flow only takes place within the transistor itself,
while electrons flow in the external circuit. However, it is the internal hole flow that leads
to electron flow in the external wires connected to the transistor.
Figure 2-10. Properly Biased PNP Transistor
2-28. PNP FORWARD-BIASED JUNCTION - Now let use consider what happens
when the emitter-base junction is forward biased (see Figure 2-11). With the bias setup
shown, the positive terminal of the battery repels the emitter holes toward the base, while
the negative terminal drives the base electrons toward the emitter. When an emitter hole
and a base electron meet, they combine. For each electron that combines with a hole,
another electron leaves the negative terminal of the battery, and enters the base. At the
same time, an electron leaves the emitter, creating a new hole, and enters the positive
2-10
TC 9-62
23 June 2005
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