2-3
is, an NPN transistor is formed by introducing a thin region of P-type material between two regions of
N-type material. On the other hand, a PNP transistor is formed by introducing a thin region of N-type
material between two regions of P-type material. Transistors constructed in this manner have two PN
junctions, as shown in figure 2-2. One PN junction is between the emitter and the base; the other PN
junction is between the collector and the base. The two junctions share one section of semiconductor
material so that the transistor actually consists of three elements.
Figure 2-2.—Transistor block diagrams.
Since the majority and minority current carriers are different for N and P materials, it stands to reason
that the internal operation of the NPN and PNP transistors will also be different. The theory of operation of
the NPN and PNP transistors will be discussed separately in the next few paragraphs. Any additional
information about the PN junction will be given as the theory of transistor operation is developed.
To prepare you for the forthcoming information, the two basic types of transistors along with their
circuit symbols are shown in figure 2-3. It should be noted that the two symbols are different. The horizontal
line represents the base, the angular line with the arrow on it represents the emitter, and the other angular
line represents the collector. The direction of the arrow on the emitter distinguishes the NPN from the PNP
transistor. If the arrow points in, (Points iN) the transistor is a PNP. On the other hand if the arrow points
out, the transistor is an NPN (Not Pointing iN).
