2-11
To further improve on the efficiency of the transistor, the collector is made physically larger than the
base for two reasons: (1) to increase the chance of collecting carriers that diffuse to the side as well as
directly across the base region, and (2) 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, IE 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 + I
C
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 emitter-base bias, and because the collector
receives most of this current, a small change in emitter-base bias will have a far greater effect on the
magnitude of collector current than it will have on base current. In conclusion, the relatively small emitter-
base bias controls the relatively large emitter-to-collector current.
Q6. To properly bias an NPN transistor, what polarity voltage is applied to the collector, and what is its
relationship to the base voltage?
Q7. Why is conduction through the forward-biased junction of an NPN transistor primarily in one
direction, namely from the emitter to base?
Q8. In the NPN transistor, what section is made very thin compared with the other two sections?
Q9. What percentage of current in an NPN transistor reaches the collector?
PNP Transistor Operation
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. To
support this different type of current (hole flow), the bias batteries are reversed for the PNP transistor. A
typical bias setup for the PNP transistor is shown in figure 2-8. 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. Thus, the base of the
PNP transistor must be negative with respect to the emitter, and the collector must be more negative than the
base. Remember, just as in the case of the NPN transistor, this difference in supply voltage is necessary 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.
