EEVblog #748 - How Do Transistors Work?

[EEVblog]

Dave explains how BJT and MOSFET transistors work at the silicon chip level.
How does a BJT transistor actually amplify current?
P and N type doping, charge carriers, conduction channel, field effect, holes and electrons, all the other good stuff.

[rs20]

Great video. There's one point that I was confused about, where you just say that the C-B depletion region shrinks as if it's obvious that would happen. But now the N and P are the other way around, wouldn't the same current flow now expand the depletion region?? But I think I figured it out while writing this post. So I'll leave it as my question and then my own answer, in case it's useful (and to get confirmation it's correct).

Question: You say that the charge carriers flooding from the emitter into the base of the BJT flow through the C-B depletion region and out the collector. But that depletion junction is P on the bottom, N on the top, that's a reverse biased diode! If I replaced the emitter N region with a metal contact, clearly that'd be a reverse biased diode and no current would flow. What's different here?

I think the answer is: A metal contact would flood the base with holes, which (according to how a diode works), would just expand the depletion region even more, and no current would flow. But in the BJT, the emitter is flooding the base with electrons. Perhaps this point could have been highlighted a bit; the P material in the base, which is normally supposed to be full of holes, has completely changed and now has an excess of electrons. And so it's temporarily a NNN device, and in turn that's why the depletion regions are gone.

Is that right?

[JackOfVA]

In the mid-60's when I was studying electrical engineering at university, we started with the Ebers-Moll model, which relates emitter current to Vbe and to think of a BJT as a voltage controlled device.

While in many cases - perhaps even most - practical design can consider a BJT as a current controlled device, the variation in HFE from device to device and with temperature can and will bite you in the backside on occasion.  If I wish to use a BJT to switch, for example, a relay on and off, it's entirely reasonable to use the worst case HFE, throw in some temperature margin and a safety factor.

[c4757p]

There will be another voltage-vs-current argument here, and I will be watching for entertainment.

*passes around popcorn* 

[Yansi]

I will just ignore that :-)

I look forward to Dave digs again into his project box and shows us some other of his projects he designed and built. I consider that much more interesting than PN-junction V/I war :-)

[rsjsouza]

There will be another voltage-vs-current argument here, and I will be watching for entertainment.

For the folks interested in creating a flame war, just read this entire thread as many times necessary until your itch is calmed... 

c4757p, sorry for my attempt to burst your bubble...

[vlad777]

Dave, you just hit a troll nerve with saying " current driven" 

[kjn4685]

Well I never went to college, but I have learned a lot from these videos. Thanks Dave

[TimFox]

No argument intended about voltage vs. current.
A forward-biased PN junction has a monotonic curve of current vs. voltage, and therefore has a monotonic curve of voltage vs. current.
It is often easier to consider the voltage as a function of current, since the voltage is approximately a logarithmic function that changes much less rapidly with respect to current and the parameters.

[MobileWill]

Thanks for the great video. Makes much more sense that other explains I have seen. I realize thinking back to old project a MOSFET would of been much better suited. I had a microcontroller pin connected to a TIP120 via a resistor of course to turn a 9V RF Camera on and off. Never worked very well.

On a side note, I don't know why anyone would give a thumbs down... and more than one at that.

[tobia]

I did go to college, but I only studied Maths and various CS subjects there. So I learn a lot from these videos too. Thanks!

I agree that some parts of the explanation about the depletion region sounded kind of vague. But I can see how it would have needed a 10-fold increase in complexity to get a 1% increase in understanding, so I won't fault Dave for staying below that threshold.

[rfeecs]

Great video. There's one point that I was confused about, where you just say that the C-B depletion region shrinks as if it's obvious that would happen. But now the N and P are the other way around, wouldn't the same current flow now expand the depletion region?? But I think I figured it out while writing this post. So I'll leave it as my question and then my own answer, in case it's useful (and to get confirmation it's correct).

Question: You say that the charge carriers flooding from the emitter into the base of the BJT flow through the C-B depletion region and out the collector. But that depletion junction is P on the bottom, N on the top, that's a reverse biased diode! If I replaced the emitter N region with a metal contact, clearly that'd be a reverse biased diode and no current would flow. What's different here?

I think the answer is: A metal contact would flood the base with holes, which (according to how a diode works), would just expand the depletion region even more, and no current would flow. But in the BJT, the emitter is flooding the base with electrons. Perhaps this point could have been highlighted a bit; the P material in the base, which is normally supposed to be full of holes, has completely changed and now has an excess of electrons. And so it's temporarily a NNN device, and in turn that's why the depletion regions are gone.

Is that right?

I'm not surprised that you are confused because Dave's explanation is not only wrong, but doesn't even make sense.

Dave says as you forward bias the base-emitter junction the depletion region shrinks and then flips around.  This is wrong.  The depletion region does shrink, but never shrinks to zero.  There is always an emitter-base depletion region and a collector-base depletion region.

Dave implies carriers can't flow through a depletion region.  This is wrong as well.

As you pointed out, his explanation of why current flows in the collector doesn't make any sense. 

The short explanation is the collector-base is always reverse biased.  There is always a depletion region there.  As the base is forward biased, electrons flow from the emitter into the base.  Because there is a positive voltage on the collector, there is an electric field that sweeps the excess electrons right through the base-collector depletion region and out of the collector.  A very few electrons in the base re-combine with holes in the base and this results in a small base current flow.

I love Dave's videos, but if you really want to understand how transistors work, I suggest you take this one for entertainment value only and look at some of the many other sources of information on the internet on how transistors work.

[retrolefty]

 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. 

[rs20]

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.

Consider a bubble rising from the bottom of a glass of fizzy drink. Note that it's far easier to think of the bubble moving up, rather than thinking about the water molecules all moving down slightly at just the right time to leave a water void that appears to be moving up. I grant you, not a perfect analogy since the bubble actually contains air, but the point remains valid I think. Similarly, in semiconductors, a "hole" is a "bubble" in the sea of electrons. If you see two snapshots, one with a hole on the left of the page, and the next with a hole on the right of the page, it's far more natural to say that the hole has moved to the right, rather than saying that a whole bunch of electrons have moved left by one atom.

In any case, your claim is completely flawed because semiconductor descriptions rely on simultaneously tracking holes and electrons -- holes and electrons have equal importance in semiconductor theory, they're both invoked together in the description of a BJT, which is hardly what you'd expect if you thought holes were just a historical mistake.  If they weren't truly distinct manifestations, why would a single explanation conflate them. If you show me a (hypothetical) picture of a piece of silicon, I can point out the free electrons, and I can point out the holes. Holes are real; distinct entities from electrons.

Conventional current vs electron flow in metals is a completely separate issue, you seem to be conflating the two. If someone were claiming that holes carry current in metals, then I'd agree with you. But no-one does that.

[retrolefty]

I'm not buying it.

in semiconductors, a "hole" is a "bubble" in the sea of electrons.

 A hole, as a construct and justification to define flow direction, is simply a single atom that is carrying a positive charge, and this 'hole' will soon be occupied, just as the atom that will fill that 'hole' is simply a negatively charged atom. Bubble in a sea of electrons, indeed weird.

 Every hole flow advocate I've run across has to hold their tongue just right, as if tweeking a muti-turn trimmer. 

[EEVblog]

Dave, you just hit a troll nerve with saying " current driven" 

Too bad for them 
Undeniable fact that BJT's have base current at the application level, and is the basic operational difference between BJT's and FET's.

[EEVblog]

On a side note, I don't know why anyone would give a thumbs down... and more than one at that.

I have serial haters. Thumbs down come in the first few minutes after upload before they even have time to watch it. Happens on every video.

[c4757p]

That being the basic operational difference is not undeniable. I'd call the basic operational difference the very-close-to-pure exponential characteristic between both V_BE and I_C at low currents and V_CE and I_C at high currents (the reason why translinear circuits work so well with BJTs, and the reason IGBTs are used at very high currents). The base current is incidental. MOSFETs are much more complicated in terms of transfer function.

Damn, I told myself I wouldn't get into this.

[EEVblog]

The short explanation is the collector-base is always reverse biased.  There is always a depletion region there.  As the base is forward biased, electrons flow from the emitter into the base.  Because there is a positive voltage on the collector, there is an electric field that sweeps the excess electrons right through the base-collector depletion region and out of the collector.

Yes, correct, my goof. Have clarified this in annotation.
The "flipping" terminology was a poor choice, "overcome" would have been much better.

[T3sl4co1l]

Methinks math-hating, religion-loving Dave has only ever used BJTs as switches, and therefore believes them purely current-operated. 

(Fuck, now there's some flamebait... if y'all don't see me again, let it be known why... )

Tim

[calexanian]

Jerry! Jerry! Jerry! Jerry! Hahahahaha.. Or if you are older and remember the Jeraldo show, whatch out for that chair! Mere Griffin and Dick Cavett never did such things. Sorry to all the non yanks who might not get these references.

[jwm_]

Great video, Something you didn't touch on for MOSFETs is why they are not 'symmetric'. For the BJT it is clear that the collector has the lightly doped region. But from simplified MOSFET drawing you used (excellent 3d davecad modelling there), it appears that source and drain are identical which can be confusing if you take the drawing literally. And maybe bring up why J-FETs can be made symmetric.

[EEVblog]

Great video, Something you didn't touch on for MOSFETs is why they are not 'symmetric'. For the BJT it is clear that the collector has the lightly doped region. But from simplified MOSFET drawing you used (excellent 3d davecad modelling there), it appears that source and drain are identical which can be confusing if you take the drawing literally. And maybe bring up why J-FETs can be made symmetric.

Yes, deliberately left out. It was already getting way longer than I wanted.

[jwm_]

Yes, deliberately left out. It was already getting way longer than I wanted.

Hey, I'd watch the extended remix where you add the humble vacuum triode at the beginning and insert the JFET in between BJT and MOSFET.

[c4757p]

Great video, Something you didn't touch on for MOSFETs is why they are not 'symmetric'. For the BJT it is clear that the collector has the lightly doped region. But from simplified MOSFET drawing you used (excellent 3d davecad modelling there), it appears that source and drain are identical which can be confusing if you take the drawing literally. And maybe bring up why J-FETs can be made symmetric.

Keep in mind that MOSFETs can be symmetric.