- The emitter-base junction of a transistor is forward biased whereas collector-base junction is reverse biased. If for a moment, we ignore the presence of emitter-base junction, then practically* no current would flow in the collector circuit because of the reverse bias. However, if the emitter-base junction is also present, then forward bias on it causes the emitter current to flow. It is seen that this emitter current almost entirely flows in the collector circuit. Therefore, the current in the collector circuit depends upon the emitter current. If the emitter current is zero, then collector current is nearly zero.
- However, if the emitter current is 1mA, then collector current is also about 1mA. This is precisely what happens in a transistor.
- We shall now discuss this transistor action for npn and pnp transistors.
(1) Working of npn transistor: Fig. 4.4 shows the npn transistor with forward bias to emitterbase junction and reverse bias to collector-base junction. The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current IE. As these electrons flow through the p-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%) combine with holes to constitute base** current IB.
- The remainder (more than 95%) cross over into the collector region to constitute collector current IC. In this way, almost the entire emitter current flows in the collector circuit. The reasons that most of the electrons from emitter continue their journey through the base to collector to form collector current are:
(i) The base is lightly doped and very thin. Therefore, there are a few holes which find enough time to combine with electrons.
(ii) (ii) The reverse bias on collector is quite high and exerts attractive forces on these electrons It is clear that emitter current is the sum of collector and base currents i.e. I
E = I
B + I
C
(2) Working of pnp transistor: Fig. 4.5 shows the basic connection of a pnp transistor. The forward bias causes the holes in the p-type emitter to flow towards the base. This constitutes the emitter current IE. As these holes cross into n-type base, they tend to combine with the electrons. As the base is lightly doped and very thin, therefore, only a few holes (less than 5%) combine with the electrons. The remainder (more than 95%) cross into the collector region to constitute collector current IC. In this way, almost the entire emitter current flows in the collector circuit. It may be noted that current conduction within pnp transistor is by holes. However, in the external connecting wires, the current is still by electrons.
Wilfykil answered the question on
August 14, 2019 at 12:35