![]() ![]() To understand transistors, first banish the word "current" from all questions, answers, and explanations. And so, what moves around inside wires? Charges. "Current" means " flow of water." To understand plumbing we must understand water and its motion. In other words, what flows in rivers? Water? Or "Current?" When you turn on a faucet and fill a bucket, did "a current" come out, so you have a bucket of current? Nope. Not "flow flow" or "flow of a flow." The flow doesn't flow: there is no flow of current. To understand transistors you first need to understand charge-flow. The term "electric current" means "flow of electric charge." A current is a flow of something. But this adds base current during operation, and since recombination centers also act as thermal carrier generators, it creates leakage current, especially at high temperature.įirst you need to get this out of the way: CURRENT DOESN'T FLOW. The only exception is some very old-school high speed power transistors, where they add gold doping (which forms recombination centers) to the base, to help drain minority carriers from the base to allow the transistor to turn off faster. 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. In practice, however, one way is much shorter (which one and why?).Yes, absolutely. ![]() In principle, the two ways are equivalent: you assume a condition, saturation or active region, and then you check for which values of \$\beta\$ the condition holds. According to what I wrote in the first two paragraphs, what's the meaning of this ratio, then? Recall also that the relationship \$I_\mathrm\$ under the assumption of point 1 and find their ratio. Recall that a BJT is saturated when both the base-emitter (BE) and the base-collector (BC) junctions are forward-biased (and not when "current flows freely from the collector to the emitter", which is a meaningless definition). ![]()
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