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I am completely unable to understand the current direction in npn transistor. figure 1

in figure 1, the red arrow indicates the electron flow(which is from emitter terminal to collector terminal). thus according to the convention, the current flow is just opposite to the direction of electron flow. therefore the current must flow from collector terminal to emitter in npn trnasistor. enter image description here

but nothing like that happens. instead, the current flows from emitter terminal to collector terminal(that is in the same direction of electron flow). why is it so? why does this happen?

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migrated from physics.stackexchange.com May 19 '17 at 21:48

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Thanks for pointing out where you got the diagram that you added to your question. (You probably should now include it inside your question, rather than leave it as a comment to another answer you got.) Here's the diagram you were asking about:

enter image description here

I think the diagram was trying to illustrate the conventional current flow on the left side and the electron flow on the right side. But they made a mistake and used a PNP schematic symbol on the left when they should have stayed with the NPN schematic symbol for both cases. If you replace the PNP on the left with an NPN symbol, then the diagram makes sense in the context of illustrating conventional current directions on the left side and electron current directions on the right.


It is hard to miss that your question has almost completely changed. So the above is what I wrote then to address the question I believed you were asking then.

The new question is different. And it would probably be better to have asked that as a new question here. Changing the question makes the answers appear disconnected and perhaps even irrelevant and that doesn't help future readers who may see a question and then some answers and be completely mystified.

As far as "current direction" goes, and at least so far as copper wiring is concerned (which only has conduction band electrons to carry current, unlike aluminum which is a little more 'nuanced'), you simply decide whether or not you are talking about positive charges moving or negative charges moving. Benjamin Franklin, centuries back, didn't know how all this worked and had an even chance of "getting it right." Despite the 50% odds, he got it wrong and decided that it was positive charges. Ever since, partly because the earlier literature made the same assumptions, we've been stuck with it.

So. Don't get mentally mired in this. Since most of the rest of the world has accepted, and is using, the old Benjamin Franklin perspective of charge flow direction all you will do is continually confuse yourself and others every time to talk with someone else if you decide you are going to be the isolated voice challenging the convention everyone else is now using. Sure, the reality may be different. But so long as you stay consistent, it doesn't matter.

So current does flow from collector to emitter in the NPN. If you accept the convention, which uses "anti-electrons" as the charge carrier. If you refuse to accept it and go your own way insisting on using electrons as your charge carrier, then sure, it goes from emitter to collector. If it helps, turn this around and look at the PNP. Here, using the usual benjamin Franklin convention, current flows from the emitter to the collector and using your electron convention it flows from the collector to the emitter. Just the opposite.

Maybe for now it's better to just accept a different idea. That in a BJT operating in the active or saturated regions the emitter current is the sum of the collector and base currents, regardless of the direction you yourself insist upon. The direction you use for your imagination doesn't matter that much, so long as you stay consistent with your approach. The problem comes only when you want to communicate with others. Then you should use whatever they are using to understand and communicate back to you. Otherwise, the conversation devolves into debating terms and nothing gets done in the end.

A good DC model for the BJT is called the Ebers-Moll model. If you are interested, go look at this EE.SE answer I gave. There, you can see the simplest (DC) BJT model that exists. (It's missing an equation that describes the variation of saturation currents over temperature, but is otherwise complete.) There are three completely equivalent and identical BJT models presented there. There is no difference between them except for their mathematical formulation and what mental perspective is chosen to be emphasized by the model. I think that if you struggle mentally to acquire those equivalent models, you will see why your debate here is rather minor in the larger scheme of things and why it is best if you just learn to use terms as others use them and move on.

I'm not telling you to forget physics, such as the fact that electrons are our current working belief about charge carriers (physics is always tentative and never absolute in its proclamations of fact.) It's a good thing to keep in mind the idea that thermally agitated electrons easily move from valence bands into the conduction band in metals and that the electron cloud model operates reasonably well in explaining a whole gamut of observed behaviors of current flow, mean free path, mean velocity, etc., in metals (and other materials, as well.) And that it explains why conductors are never transparent, too. Lots of good stuff arrives from knowing that detail. But it is not important in electronics discussions that you stick to your guns and insist that current flow is about electrons. You can just as well say that it is due to anti-electrons and be just as able to explain and design circuits.

So you really need to let loose on this issue and go with the conventional approach. (Or else you will always be swimming upstream of convention.)


The currents inside a BJT are a bit more complicated than what appears on the outside, where wires connect up. See the diagram below from page 61, section 3-2, of Jacob Millman's "Microelectronics," 1979. (The diagram shows a PNP transistor.) But none of this applies when just wiring up a BJT into a circuit -- as such details are hidden from you.

Page 61, Section 3-2, Jacob Millman, "Microelectronics," 1979

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  • \$\begingroup\$ to avoid confusion in the OP, it has to be said that Millman uses a convention where (conventional, not electron) currents are positive when entering the device, no matter what. So, in the normal functioning of the above PNP in active mode, Ie would be positive (that is, directed as the arrow) while Ib and Ic would be negative (that is, directed opposed to their respective arrows). \$\endgroup\$ – Sredni Vashtar May 19 '17 at 23:46
  • \$\begingroup\$ @SredniVashtar, That convention is very common. It shouldn't have to be mentioned specially. It's called passive sign convention and has also been asked about here several times. \$\endgroup\$ – The Photon May 20 '17 at 0:50
  • \$\begingroup\$ Yep, but the OP was struggling in understanding current flow, meaning s/he was assuming the (absolute) current was actually flowing in the sense indicated by the arrow. It was worth mentioning the picture you posted used another convention. \$\endgroup\$ – Sredni Vashtar May 20 '17 at 9:18
  • \$\begingroup\$ thanks mate for giving me clear idea regarding conventions. \$\endgroup\$ – Saad Anwar May 20 '17 at 9:55
  • \$\begingroup\$ @SaadAnwar None of my addition was intended to confuse the OP, nor was it intended to indicate directions, conventional or otherwise, of currents. It was only intended to make the OP aware of the fact that when discussing BJTs the OP might on occasion encounter discussions about minority and majority currents and I wanted to make sure the OP realized these are "in-house technical" discussions and do not impact things once metalization/copper is reached. Just a note for awareness, is all. Not explication. (Also, this came from 'physics' so a nod here seemed appropriate.) \$\endgroup\$ – jonk May 20 '17 at 16:50
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The direction of the arrow in the transistor symbol (between base and emitter) tells you the direction of current:

PNP and NPN transistor

Note that "electron current" is the opposite direction from "conventional current"

Perhaps the site you used (can you share a link?) was talking about the directions in which the electrons flow?

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  • \$\begingroup\$ allaboutcircuits.com/textbook/semiconductors/chpt-4/… this is the link of the site which i was referring to. the site was not talking about the electron flow but the current flow. \$\endgroup\$ – Saad Anwar May 19 '17 at 22:19
  • \$\begingroup\$ exactly that's my question why is the conventional current opposite to the electron current? what's the reason behind it? \$\endgroup\$ – Saad Anwar May 19 '17 at 22:22
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    \$\begingroup\$ @SaadAnwar, that question has been asked answered (by me and others) many times. See, for example, this answer \$\endgroup\$ – Alfred Centauri May 19 '17 at 22:36
  • \$\begingroup\$ thanks mate for the help. it was really confusing me, at least now i know the difference between the conventional current and electron current. \$\endgroup\$ – Saad Anwar May 19 '17 at 22:39
  • \$\begingroup\$ @SaadAnwar unfortunately allaboutcircuits.com teaches numerous 'technician misconceptions.' If you intend to go into engineering as a career, it's best to steer clear of any sites which use oversimplification and "Lies To Children." Instead, bite the bullet and get the real stuff from physics websites, college-level sites, and engineering and semiconductor texts. Transistor and semiconductor concepts are hard enough to grasp, without having to unlearn an entire set of wrong ideas. (In other words if you feel confused, the real problem might be the websites which use wrong explanations.) \$\endgroup\$ – wbeaty May 20 '17 at 1:43
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Someone has succesfully feeded to you some total disiformation.

Everything is not bullshit.Your this image

enter image description here

shows the flowing directions of the electrons and also shows which loop has higher flow rate electrons per second, if the transistor is used as usually (= the base current controls the collector current)

But the direction of the current is the opposite of your arrows.

The general agreement on "the current has the opposite direction of the electron flow" should be used also here. That agreement was done when there was no evidence of the existence of so called electrons. Instead, the electric current seemed to carry metal through liquids. That phenomena was quite believable, so the direction of the current was nailed to be the opposite of the direction of the electrons. When the electrons finally were accepted due the new accurate measurements and plausible theories, the electric industry was already so massive that the current direction convention had become untouchable.

The arrow in the emitter tells the current direction. Hopefully you have noticed that the currents in the transistor come from the external sources. The transistor is only a way to those currents, altough quite complex one when compared to wires or resistors.

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  • \$\begingroup\$ do you have handy a reference for the origin of the sign convention? With "seemed to carry metal through liquids" I suppose you are referring to ions in a solution. I am curious about what where these first experiments and who did them (Faraday, perhaps?). \$\endgroup\$ – Sredni Vashtar May 21 '17 at 22:39
  • \$\begingroup\$ @SredniVashtar No single reference. Most basic concepts took very long time to get stabilized. The opposite directions of the current and the motion of electrons was actually caused by Benjamin Franklin who selected + and -. He made some plausible experiments. They made believable that electric charge can be something extra (+) or the lack of that something (-). He selected arbitarily which is + and which - by the materials that he used to produce the frictional electricity. Once accepted, it stayed. Faraday's experiments with electrolysis fitted perfectly, but the foundation was Franklin's. \$\endgroup\$ – user287001 May 22 '17 at 0:03

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