NPN
The 2N3053 transistor is known as an NPN transistor because of the fact that a thin layer of P-type semiconductor material is sandwiched between two layers of N-type semiconductor material. The construction and circuit symbol of an NPN transistor are shown in Figure 8.10(a). You may have worked out that there is another way to sandwich one type of semiconductor material between two others — and if so you would also have worked out that such a transistor would be called a PNP transistor. Figure 8.10(b) shows a PNP transistor construction and its circuit symbol. The only difference in the circuit symbols of both types is that the arrow on an NPN transistor’s emitter points out and the arrow on a PNP transistor’s emitter points in.
The emitter arrow of either symbol indicates direction of base current and collector current flow. So from the circuit symbols we can work out that base current in the NPN transistor flows from base-to-emitter, while in the PNP transistor it flows from emitter-to-base. Likewise collector current flow in the NPN transistor is from collector-to-emitter and from emitter-to-collector in the PNP transistor.
Figure 8.10 The internal construction and circuit symbols for NPN and PNP transistors
Knowing this and comparing the PNP construction to that of the NPN transistor we can further work out that a tiny emitterto- base current (still called the base current, incidentally) will cause a much larger emitter-to-collector current (still called the collector current). This is illustrated in Figure 8.11. The ratio of collector current to base current of a PNP transistor is still the current gain. In fact, apart from the different directions of currents, a PNP transistor functions identically to an NPN transistor. As we’ve started our look at transistors with the use of an NPN transistor, however, we’ll finish it the same way.
Figure 8.11 An NPN has the same effect as a PNP transistor, but in the opposite direction
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