PNP Transistor Operation
Author: Leonard Krugman
A P-N-P junction transistor, shown in Fig. 2-7, is formed by sandwiching a thin layer between two relatively thick P areas. As in the case of the N-P-N junction transistor, the electrode on the left is designated the emitter, the electrode on the right is designated the collector, and the common electrode is designated the base. However, the polarities of the potential hills formed are opposite those formed in the N-P-N junction transistor. In order to adhere to the general rules of biasing the emitter in the low resistance direction, and biasing the collector in the high resistance direction, the polarities of the external bias batteries are also reversed.
Fig. 2-7. Basic P-N-P transistor.
Conduction in the P-N-P junction transistor is similar to that in the N-P-N type. The holes in the P emitter region are repelled by the positive battery electrode toward the P-N junction. Since this potential hill is reduced by the emitter bias, a number of holes enters the base N area. A small number of holes (approximately five percent) is lost by combination with electrons within this area, and the rest move toward the collector, aided by the action of the collector-base potential hill. As each hole reaches the collector electrode, the collector emits an electron to neutralize the hole. For each hole that is lost by combination within the base or collector areas, an electron from one of the covalent bonds near the emitter electrode enters that terminal, thus forming a new hole in the vicinity of the emitter. The new holes immediately move toward the junction area. Thus a continuous flow of holes from the emitter to collector is maintained. It is evident that in both types of junction transistors discussed, the collector current is less than the emitter current by a factor proportional to the number of hole-electron recombinations that take place in the base junction area.
This analysis of the junction transistor leads to the following general observations:
- The major current carriers in the N-P-N junction transistors are electrons.
- The major current carriers in the P-N-P junction transistors are holes.
- The collector current in either type of junction transistor is less than the emitter current because of the recombinations of holes and electrons in the base junction area. As an example, a typical rate of recombination is five percent. If an emitter current of one milliampere is assumed, the collector current is 1 ma. — (1 ma. × .05) = 0.95 ma.