The PNPN Transistor

Author: Leonard Krugman

Fig. 2-10. Basic P-N-P-N transistor construction.

Figure 2-10 illustrates the construction of the P-N-P-N junction transistor. This transistor, unlike the P-N-P or N-P-N junction transistor, is capable of a current gain. For satisfactory operation, both of the central P and N regions must be narrow.

In operation, the holes move in the direction from emitter to collector, but are trapped by the third potential hill in the collector area. The holes pile up at this barrier, and their cumulative positive space charge reduces the effect of the potential hill. As a result, electrons from the collector area encounter a decreased resistance at the junction and are able to flow into the central P region. Some electrons are lost through combinations with holes, but most of them, aided by the action of potential hill number 2, move into the middle N region and enter the base.

The P-N-P-N construction, because of the space charge effect of the holes, allows the current gain to reach values in the vicinity of 20. In comparison it must be remembered that the current gain of the prototype junction transistor is inherently limited to values less than one.