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Topic: NPN transistor (Read 2835 times) previous topic - next topic


Hello, i was hoping you guys can help me with some understanding of the NPN BJT transistor. I am reading Practical Electronics by Nigel P. Cook (fantastic book), but i had a question which was consfusing me a bit.

I read the background theory of how the doping works, the electron flow due to hole flow and electron flow, etc... I believe i understand the operation, especially with normal diode (basically act as a check valve, allow one way current). Now the problem i have with the NPN is as follows, in the book they draw the NPN as having 2 back to back diodes. Both ends at the base are the Anode, and the ends (collector,emmiter) are Cathode.

Now when the transistor is fully saturated, both junctions are forward biased (check valves are open) and from the schematic it makes sense that the transistor allows full current. Similarly, when it is cut off, both junctions are reverse biased (check valves closed) and no current flows. Makes sense....

Now when it comes to active mode: the base-emiiter junction is forward biased and allows current to flow between base and emmiter (makes sense). Now the thing that does not make sense to me is this: in active mode, the collector-base junction is reverse biased, so how is it allowing current to flow? From learning about stand-alone diodes, when it is reverse biased it gives no current other than the leak current (which is really tiny), and it usually happens at Voltages that are real high ~10-50 V. So how does a large current flow through the main branch (emmiter-collector) even though that diode is reverse biased? Would it not only allow the leak current only, which is barely nothing?

The book describes the collector-base as being OFF, base-emmiter ON.

I am trying to understand what is going on, and i think i can come up with something that might explain it, but im not sure. When the base-emmiter is foward biased, electrons enter the base junction and make the depletion region smaller. But because the collector is attached to +, the electrons in N go towards it, leaving positive holes near the junction with the base, and these holes then can be filled with the electrons that just entered the base? Is that why current can go through?


You are making the transition from discussing "electronics" to discussing "semiconductors". The Wikipedia entry has a good introduction of how electrons can flow through this seemingly reverse-biased "collector-to-emitter diode":


A "model" for an NPN transistor is two diodes back-to-back, as you describe, but you cannot just put two diodes back-to-back and have yourself a transistor. You really need to understand how semiconductors work at the physical level, as described in the Wikipedia article above (briefly), and in texts on semiconductor theory. For example, electrons are actually going in the completely opposite direction from what you describe (emitter to base and collector), but from an "electronics" viewpoint we don't make the distinction (and it doesn't matter). But once you start discussing "semiconductors" then it matters.

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Basically the difference between an NPN transistor and two PN diodes back to back is that the charge carriers that make it across the emitter-base junction travel far enough to be inside the base-collector junction and see the collector potential before having much of a chance to re-combine.  They get whisked away to the collector by the strong electric field within the base-collector depletion zone.

The base region of a transistor has to be very thin to get this effect.  If the carriers recombine in the base they just form part of the base current and don't contribute to collector current.  For an NPN transistor with a current gain of 100 the chance an electron from the emitter makes it to the collector is 99% (and only 1% that it finds a hole from the base to combine with).  Typically the base region is very lightly doped to reduce the chance of recombination.
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An NPN transistor can be "viewed" as two junction diodes connected together but the p region is much thinner than the two N regions. When the transistor is under bias or in active region the BE junction is forward biased, allowing the majority charge carriers in the emitter (n) to inject into the base. When the transistor is under bias or in active region the BC junction is reverse biased, preventing the majority charge carriers in the collector (n) from injecting into the base. So, all charge carriers come from the emitter (thus the name emitter). Why do they make through a reverse biased BC junction? Well, over 90% of the charges make through, not because they are up against any obstacles, because they are not. The electric field in the reverse biased BC junction helps the electrons move towards the collector, not prevent them.

The reason a semiconductor diode only conducts one way is because its charge carriers (majority) only move one way when forward biased, eg. holes go from anode to cathode, or electrons from cathode to anode, much like the vacuum tube diode. If you reverse bias, majority charge carriers are pushed the wrong way by the reverse bias while the minority charge carriers are pushed the current way to conduct. But there are not enough minority charge carriers so conduction is next to nothing. If you hypothetically inject a bunch of say electrons in the p region of a reverse biased p-n diode, with a syringe, you will see significant conduction until the electrons are used up or recombined with holes.

The reason a transistor conducts even when the CB junction is reverse biased is because of the injected charge carriers from the emitter do the conduction, through the reverse biased junction. If left alone without emitter to inject charges, the reverse biased CB junction will simply not conduct. That is why the base is made much less doped and much thinner than the emitter so that the injected charges make to the CB junction without much loss and do the conduction.

It's like the USA is in such urgent need for STEM people but they themselves are so reverse biased from doing STEM, they have to inject STEM people from other countries to keep their high-tech industry flowing. If foreign talents stop coming to the USA, their high-tech industry will likely suffer, under the current negligence of STEM education of the public.  XD
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