The bias batteries in the figure 5 have been labeled V CC for the collector voltage supply, and V BB for the base voltage supply. Also notice the base supply battery is quite small, as indicated by the number of cells in the battery, usually 1 volt or less. However, the collector supply is generally much higher than the base supply, normally around 6 volts.
This difference in supply voltages is necessary to have current flow from the emitter to the collector. Figure 5: npn transistor operation is basically the action of a relatively small emitter-base bias voltage controlling a relatively large emitter-to-collector current.
The current flow in the external circuit is always due to the movement of free electrons. Therefore, electrons flow from the negative terminals of the supply batteries to the n-type emitter. This combined movement of electrons is known as emitter current I E. Since electrons are the majority carriers in the n- material , they will move through the n- material emitter to the emitter-base junction.
With this junction forward biased, electrons continue on into the base region. Once the electrons are in the base, which is a p-type material, they become minority carriers. Some of the electrons that move into the base recombine with available holes. For each electron that recombines, another electron moves out through the base lead as base current I B creating a new hole for eventual combination and returns to the base supply battery V.
The electrons that recombine are lost as far as the collector is concerned. Therefore, to make the transistor more efficient, the base region is made very thin and lightly doped. This reduces the opportunity for an electron to recombine with a hole and be lost. Thus, most of the electrons that move into the base region come under the influence of the large collector reverse bias. This bias acts as forward bias for the minority carriers electrons in the base and, as such, accelerates them through the base-collector junction and on into the collector region.
Since the collector is made of an n-type material, the electrons that reach the collector again become majority current carriers. Once in the collector, the electrons move easily through the n material and return to the positive terminal of the collector supply battery V CC as collector current I C.
The forward bias causes the holes in the p -type emitter to flow towards the base. As these holes cross into n -type base, they tend to combine with the electrons. 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.
Importance of transistor action. The input circuit i. As we have seen, the input emitter current almost entirely flows in the collector circuit.
Therefore, a transistor transfers the input signal current from a low-resistance circuit to a high-resistance circuit. This is the key factor responsible for the amplifying capability of the transistor. Recommended Articles. The emitter is moderately doped, and the collector is heavily doped. The circuit diagram of the NPN transistor is shown in the figure below.
The collector and the base circuit is connected in reverse biased while the emitter and base circuit is connected in forward biased. The forward biased is applied across the emitter-base junction, and the reversed biased is applied across the collector-base junction. The emitter of the NPN transistor is heavily doped. When the forward bias is applied across the emitter, the majority charge carriers move towards the base.
This causes the emitter current I E. The electrons enter into the P-type material and combine with the holes. The base of the NPN transistor is lightly doped.
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