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Bjt transistor beta infinite meaning9/3/2023 ![]() ![]() So typically a transistor is designed so that diffusion wins easily. The competition between diffusion to the collector and recombination in the base governs the overall efficiency of emitter injection into the collector. ![]() Edit: which is how you can allow the collector-emitter current to be much larger than the base-emitter current. So the electrons that come from the emitter in an NPN aren't automatically sucked up by the base lead, they would have to recombine with a base-region majority carrier (hole) to result in base current. ![]() To understand this principle, it's key to realize that when you make a metallic contact to a semiconductor, the contact is effectively only made to the majority carrier reservoir. So the heavy emitter doping causes a large electron current to flow to the base and diffuse to the collector, while the light base doping causes a much smaller hole current to flow to the emitter from the base, even though both currents result from the same bias voltage. When you have a PN junction, there's nothing that dictates that the number of holes crossing one way and electrons crossing the other way under forward bias need to be equal. The key is that the emitter is doped more heavily than the base. Here there's an electric field that drags them into the collector, and they become majority carriers in this context and can travel straight to the collector terminal. But since the base region is so thin, while they're diffusing around waiting to recombine, many of them bump into the base-collector deletion region. ![]() The emitter region electrons also cross the junction as a result of the applied voltage. So in the case of an NPN, the holes in the base region (which are majority carriers in this context) travel across to the emitter, where they are now minority carriers, and eventually recombine and result in a current to the base lead from the base. Charge carriers flow from both sides, since the barrier is lowered for both holes in the P region and electrons in the N region. What happens is that the base and emitter terminals apply a voltage to this junction, and as a result the depletion region is reduced and depletion barrier is lowered until carriers start to flow. The fundamental core of the BJT is a assymetrically doped base-emitter junction. The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the collector-base junction increases, which net current is called I C B 0.There is a lesser chance for recombination within the "smaller" base region.The emitter–base junction is unchanged because the emitter–base voltage is the same.īase-narrowing has two consequences that affect the current: The narrowing of the collector does not have a significant effect as the collector is much longer than the base. The principle governing these two widths is charge neutrality. The collector depletion region also increases under reverse bias, more than does that of the base, because the collector is less heavily doped than the base. Under increased collector–base reverse bias, the lower panel of Figure 1 shows a widening of the depletion region in the base and the associated narrowing of the neutral base region. The neutral emitter and collector regions are dark blue and the depleted regions hashed light blue. active) base is green, and the depleted base regions are hashed light green. A greater reverse bias across the collector–base junction, for example, increases the collector–base depletion width, thereby decreasing the width of the charge carrier portion of the base. Early, is the variation in the effective width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage. The Early effect, named after its discoverer James M. The Early voltage ( V A) as seen in the output-characteristic plot of a BJT. Top: NPN base width for low collector–base reverse bias Bottom: narrower NPN base width for large collector–base reverse bias. Variation in the effective width of the base in a bipolar junction transistor Figure 1. ![]()
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