NPN- Transistor Operation
|The forward-biased junction in an npn transistor|
We now have a properly biased npn transistor.
In summary, the base of the npn transistor must be positive with respect to the emitter, and the collector must be more positive than the base.
|The junctions in an npn transistor.|
npn forward-biased junction
An important point to bring out at this time, which was not necessarily mentioned during the explanation of the diode, is the fact that the n- material on one side of the forward-biased junction is more heavily doped than the p material. This results in more current being carried across the junction by the majority carrier electrons from the n- material than the majority carrier holes from the p material. Therefore, conduction through the forward-biased junction, as shown in figure 3, is mainly by majority carrier electrons from the n- material (emitter).
|Currents around the forward-biased junction in an npn transistor.|
npn reverse-biased junction
|The reverse-biased junction in an npn transistor.|
npn- junction interactionThe bias batteries in the figure 5 have been labeled VCC for the collector voltage supply, and VBB 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.
|npn transistor operation is basically the action of a relatively small emitter-base bias voltage controlling a relatively large emitter-to-collector current.|
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 VCC as collector current (IC).
To further improve on the efficiency of the transistor, the collector is made physically larger than the base for two reasons: (1) to increase the chance of collecting carriers that diffuse to the side as well as directly across the base region, and (2) to enable the collector to handle more heat without damage.
|Total current flow in the npn transistor|
In summary, total current flow in the npn transistor is through the emitter lead. Therefore, in terms of percentage, IE is 100 percent. On the other hand, since the base is very thin and lightly doped, a smaller percentage of the total current (emitter current) will flow in the base circuit than in the collector circuit. Usually no more than 2 to 5 percent of the total current is base current (IB) while the remaining 95 to 98 percent is collector current (IC). A very basic relationship exists between these two currents:
IE = IB + IC
In simple terms this means that the emitter current is separated into base and collector current. Since the amount of current leaving the emitter is solely a function of the emitter-base bias, and because the collector receives most of this current, a small change in emitter-base bias will have a far greater effect on the magnitude of collector current than it will have on base current. In conclusion, the relatively small emitter-base bias controls the relatively large emitter-to-collector current.