EEVblog #748 - How Do Transistors Work?

[rs20]

Remember how I said the resistor gives you the same amount of voltage on the other side, just lower current? It just lowers and limits the amperage that can go through it?

Stopped there.

Good move, you saved yourself from hearing "A transistor is just a resistor. A transistor is just a resistor. A transistor is just a resistor." At this point I pinchoff-ed the video 

[G0HZU]

I thought that Dave’s explanation was fine up to the moment where he skipped over the important bit ie. how the gain is actually generated.

Agreed.

It all went a bit wishy washy as in 'bloke from the pub' once the BJT operation was described in detail with a few errors and (as you say) he managed to miss the most important bit of all.

I only watched it up to the end of the section on the BJT.

What I found surprising was how some people commented that it was the best video/tutorial on this subject they had ever seen. However, I suspect that Dave could show a video of himself describing a turd on a string and still get high ratings. i.e. the best turd on a string video they had ever seen

[Rasz]

Remember how I said the resistor gives you the same amount of voltage on the other side, just lower current? It just lowers and limits the amperage that can go through it?

Stopped there.

Good move, you saved yourself from hearing "A transistor is just a resistor. A transistor is just a resistor. A transistor is just a resistor." At this point I pinchoff-ed the video 

Im sure both of you love hearing about moving holes and doping, but reality is novices do not understand any of it. Resistor is easy to understand, comparing transistor to resistor you can control is brilliant.

[andiz]

I think there is one thing to note regarding FETs, that is misleadingly (especially for newbies) taught a lot in literature:
FET's in most publications are described as voltage controlled, but this is only correct at the physical level of the conductive channel itself.
At the application level keep in mind you have to charge/discharge a capacitor (the gate capacitance) to get the needed voltage to build up an electric field to change the conductivity of the channel. Keep in mind you can not simply switch off an enhancement FET by switching off the control voltage, you have to actively discharge the gate capacitance to turn it off.
In low frequency applications the term of a "voltage controlled" transistor may be approximately correct (I nevertheless think this is misleading), but in high-frequency switching-applications with several 10s ore 100s of kHz a lot of driving current may be needed to change the charge of the gate capacitance.
I know about a former fellow student who burned his power-MOSFET circuit several times because he relied on the definition of a FET being a voltage controlled device.
So in fact FETs are not voltage controlled, they are charge controlled.

Best regards,
Andreas

[retrolefty]

Remember how I said the resistor gives you the same amount of voltage on the other side, just lower current? It just lowers and limits the amperage that can go through it?

Stopped there.

Good move, you saved yourself from hearing "A transistor is just a resistor. A transistor is just a resistor. A transistor is just a resistor." At this point I pinchoff-ed the video 

Im sure both of you love hearing about moving holes and doping, but reality is novices do not understand any of it. Resistor is easy to understand, comparing transistor to resistor you can control is brilliant.

 To be fair to the original inventors of the transistor, it's very name gets to a simple construct that even beginners should understand without having to snow them or make up strange analogies or inventing 'bubbles'.

 The transistor name is a contraction of 'transfer of resistance'. That is by controlling the resistance of the BE junction one can control the resistance of the CE current path. Building from that start should make for a pretty simple to grasp explanation of how a transistor works. No need to get into the chemistry of doping material or junction construction. 

[number33]

but in high-frequency switching-applications with several 10s ore 100s of kHz a lot of driving current may be needed to change the charge of the gate capacitance.

So in fact FETs are not voltage controlled, they are charge controlled.

Andiz is absolutely correct.  For designers of switch mode power supplies, a major consideration is getting enough drive current into the main switching MOSFET in order to turn it on and off sufficiently fast.  This is why MOSFET drivers exist and why switch mode supply ICs have output current capabilities in the Amps range when it only takes microAmps at dc.  Gate charge is a parameter on the data sheet for a good reason.

[T3sl4co1l]

What's more, BJTs are also charge controlled, at least if you want to use them at the peak of speed.  (Calling out "charge" versus "voltage" is a fairly useless tweak, because to put charge on the terminal, you must change the terminal voltage -- driving it from a resistor divider, for example, doesn't count, for the reason that, you aren't changing the voltage on the input node itself quickly, due to its capacitance.)

The most easily forgotten quirk of BJTs is that they store charge, exactly like a battery stores charge.  Just extremely faster (microseconds, not hours), and faster self-discharge (tens of microseconds).  The discharge curve even looks like a battery: if you fully charge the B-E junction of a BJT, then let go, it'll quickly drop down from the 0.7-0.9V it had in forward bias (or forward charge, if you will), then take a long time passing the 0.5-0.7V range (where most of the charge is stored -- exponentials being what they are), then collapse in the <0.5V range.  At the same time, the collector voltage doesn't budge until the 'collapse' phase.  A B-E resistor, or anything to sink current from the junction ("clearing stored charge"), quickens turn-off substantially.

Tim

[c4757p]

Remember how I said the resistor gives you the same amount of voltage on the other side, just lower current? It just lowers and limits the amperage that can go through it?

Stopped there.

Good move, you saved yourself from hearing "A transistor is just a resistor. A transistor is just a resistor. A transistor is just a resistor." At this point I pinchoff-ed the video 

Im sure both of you love hearing about moving holes and doping, but reality is novices do not understand any of it. Resistor is easy to understand, comparing transistor to resistor you can control is brilliant.

The sentence I quoted is wrong, not just oversimplified.

[Kleinstein]

I very much agree with the term charge controlled for both MOSFETs and BJTs. The difference is that in a BJT the charge is intrinsically decaying (one the µs time scale) and leaking through the forward biased BE junction.

I thing there were quite a lot of mistakes in Daves explaination of the BJT. Teh One from number33 is much closer to reality.

To me the simple picture of the NPN BJT is, that the emitter is injecting electrons to the base. Due to lower doping of the base, only few holes are injected to the emitter. The base to collector junction can be very much seen like a photodiode. Electrons in the base, close to the junction are collected by the collector and allow current flow. So the electron injected by the emitter act very much like electrons generated by light. Just like the quatum efficiency of photodiodes can be rather close to 1, most electrons end up at the collector and not the base. This picture also explains why the collector - Emitter saturation voltage can be smaller than 0.6 V. The low doped layer at the collector is more of a detail to get a resonable high voltage rating - so the simple picture may work without it.

The one thing that this simplified model does not explain is the function of a photo-transistor however.

[rfeecs]

I agree number33's explanation is very good.  Especially because he mentions that the carriers in the base move into the collector junction because of diffusion and are swept through because of the electric field (drift).  It might have been easier for Dave to start out with a video about "How do diodes work", and talk about drift and diffusion and recombination.  Not very difficult concepts.  After covering PN junctions, the bipolar transistor would have been easy.

The collector-emitter saturation voltage is just Vbe - Vbc, where both junctions are forward biased when the transistor saturates.

[G0HZU]

I agree number33's explanation is very good.  Especially because he mentions that the carriers in the base move into the collector junction because of diffusion and are swept through because of the electric field (drift).  It might have been easier for Dave to start out with a video about "How do diodes work", and talk about drift and diffusion and recombination.  Not very difficult concepts.  After covering PN junctions, the bipolar transistor would have been easy.

I agree. As a student I was first taught how a diode works, how to model and predict its behaviour and then how to apply this to the BJT when configured in common base with the base terminal grounded. The similarity in behaviour to a diode here is easy to show and to model in terms of input impedance and emitter current vs Vbe etc. Also the transistor 'alpha' in common base mode is easier to explain and understand in terms of how nearly all of the electrons flow from emitter through the base region and then get swept into the collector. The potential benefits this offers in terms of signal amplification are easy to demonstrate at this point. The transconductance of the BJT is easier to explain in common base. But that's just my opinion of course

[ez24]

Thank you very much for the lesson. I'm a mathematician, not a physicist, and I'm starting with semiconductor physics for fun (going through this magnificent online reference here).

http://ecee.colorado.edu/~bart/book/book/chapter1/ch1_2.htm#1_2_1

Great now I can start understanding the formulas in the Big Bang Theory tv show.

Now for a serious question - does anyone get a headache reading this?  (I do after about 20 minutes)  I do not think there are any pills that can help (seriously).  I was got a stack of Science magazines from the library and I tried really really hard to read them but always ended up with bad headaches.  So I was wondering if it is just me or does someone else have this problem?

[orolo]

Now for a serious question - does anyone get a headache reading this?  (I do after about 20 minutes)  I do not think there are any pills that can help (seriously).  I was got a stack of Science magazines from the library and I tried really really hard to read them but always ended up with bad headaches.  So I was wondering if it is just me or does someone else have this problem?

Doesn't happen to me normally. Maybe you try to cover too much material too fast. Unless I'm under pressure, I tend to digest physics material like that quite slowly, repeating calculations and derivations in a notebook, and referencing results I'm not sure about to textbooks. If I'm interested, I can keep doing that for hours without pause. That way, I eventually form a picture of the subject that makes sense to me. For example, the section you linked is a very quick review of basic quantum mechanics (first course in a physics degree); you can go through that very quickly if you already know it. Otherwise, trying to assimilate these contents is better done with a good textbook and liberal doses of free time.

[John Coloccia]

I still consider the concept of "hole flow" to be a fraud. Just made up to try and justify the original explorers of electronics incorrectly guessing that current flowed from positive to negative. After science started to actual discover and understand the structure of atoms and how it allows current to flow via electrons in it's valence layer, it become clear to me that the only thing moving were electrons. So holes were invented to support hole flow so they could continue to say that current (in the conventional definition) flows from positive to negative. But sense they still hang on to the fraud of 'conventional flow direction' in most all EE programs, we are still stuck with it.

 So when a newcomer to electronics asks what direction does current flow, why must we honestly answer that electrons flow - to +, but then go on to explain why all the semiconductor arrow symbols point against this very flow because of holes?  Sounds like pulling out of the burning bush or virgin birth to explain what they didn't originally understand well enough to state what actually moves (flows) and in what direction.

There are just too many holes in the concept of hole flow. 

I'll probably be trolled to shut my hole, but there it is. 

The "holes" are like quasiparticles.  They exist in the sense that they behave as though there really was a positive particle there with the same properties as a hole.  If you don't talk about holes flowing, then instead you're forced to talk about all of the other electrons surrounding the hole moving the other way, so that the absence of an electron in effect moves.  They are 100% equivalent viewpoints, but it's much simpler and more intuitive to talk about the "hole".

It's like when you talk about the group velocity of a wave.  Well, there's only one real velocity, and that's the velocity of some individual particle, but it makes sense to treat a waveform as it's own thing, even though it only exists as a logical construct.

But really, it's not any different than how we use the word "hole" in everyday life.  For example, if you need to bolt a few things to each other, it's frustrating, isn't it?  Somehow, you have to get this bolt to line up with all of these different holes, all at the same time.  What do you think to yourself?  "I have to get all of these holes lined up", and you move the holes around to line them up.  Well, you could also think to yourself "I need to move all of this metal out of the way to find a place where there's no metal", but no one would ever think that.  It's equivalent, but it's much simpler to think in terms of moving the hole.  It's exactly the same thing with semiconductors.  If the hole moves, the surrounding electrons move...if the surrounding electrons move, the holes move.  You pick whichever model is easiest for the task at hand.