How Transistor works - verifying

[Studiot]

Did you take note of who the original intended audience was?

That too is a very good point.

 

[wbeaty]

Comparing this website with the one I linked to in post#21 of this thread I think that 'Ratch' and 'Ratchit' are one and the same person. I note he signs himself Ratch in cabraham's E-Tech thread, although his handle is 'ratchit'.

Yep probably the same, but probably avoiding ban by mods. Following your links, I note a constant parade of odd questionable events there. Mods freely using personal attacks? And misusing moderator power while participating in battles with users? Multiplying sockpuppets and constant deceptive practice. The usual anti-trolls rule against namecalling is missing.

In June 2008 he started the thread I linked to (post#21) by referring to your site (amasci) as proof that 'transistors are voltage controlled not current controlled'.

<red sweaty face>BUT THEY ARE!</red sweaty face>

But seriously, my article originally was written in response to this question: what's a good way to explain the BJT to my grandmother? Or equivalently, how can we simplify the BJT down to the point where its operation becomes obvious to anyone?

Here's an appropriate comment: http://www.scottberkun.com/blog/2006/there-are-two-kinds-of-people-complexifiers-and-simplifers/"

Of course that question is entirely different from this one: what's the single best transistor model, the One True Path which all Proper chip engineers should use? Never ask that one, since it's a recipe for endless "Swiftian Battles."

A Swiftian battle is where two populations are driven to nearly-murderous rage over disagreement regarding the One True Way to crack an egg. In Gulliver's Travels, the two Lilliputan countries were slaughtering each other because every Proper citizen knows that morning breakfast eggs must be cracked on the pointed end ...and those despicable Outsider Others who disagree, they're disgusting heretics who need prompt incineration. :) Johnathan Swift clearly was well acquainted with the academics of his time. Tiny little men? Best way to "crack an egg?" Obvious rage and a desire to silence the blasphemous opponents?

My transistor article inadvertently triggers one of these battles when I wrap the original goal inside another one: "How do they REALLY work." The flames break out. Even Win Hill of Art of Electronics arrived and put in his two cents.

 

[wbeaty]

I also agree that the weight of practical experience of countless engineers and scientists ove the years have found current control configurations to be the most useful.

Ooo, here's an idea and a trick question inspired by the transistor war...

What if Ib was actually zero in BJTs?

What if hfe didn't exist? We'd be screwed, right? We could no longer use the linear relation between Ib and Ic. The diode exponential function would become part of everything. To prevent extreme distortion, only very small Vbe signals would be allowed.

Right? :)

 

[Studiot]

I think I have already described, here or in my references, the situation when Ib=0.

It is not my purpose to discuss the inner working of other sites, except to observe that progress is generally facilitated by goodwill. With goodwill moderation is often self moderation.

I have described, as has Claude, methods for calculating the variation of Ic with frequency by considering current or charge.

I have further described, in later posts in my link, an experiment which shows how to set up a transistor so that this change in Ic with frequency is independent of base voltage.

I asked Ratch for a derivation of equations showing how to calculate with the voltage only model in this case, with no response.

You have the same opportunity here.

I am not wedded to any particular transistor model. surely the 'best model' is the one which yields the required information with the least calculative effort. As such there is no best model, since models and calculations, correct at 2Hz will not suffice for 2GHz. A qualitative model such as the man cranking the variable resistor is very useful for achieving uderstanding but little use in real circuit calculations. I usually offer a man turning a tap (faucet) to control the flow.

go well

 

[cabraham]

Ooo, here's an idea and a trick question inspired by the transistor war...

What if Ib was actually zero in BJTs?
What if hfe didn't exist? We'd be screwed, right? We could no longer use the linear relation between Ib and Ic. The diode exponential function would become part of everything. To prevent extreme distortion, only very small Vbe signals would be allowed.

Right? :)

If Ib was zero, it would NOT be a bjt! How can an n-p-n structure have Ib = 0 when b-e jcn is forward biased? An E field exists in b-e region oriented so that holes move from base to emitter & electrons move from e to b. The n-type emitter has an abundance of free electrons, they being the majority carrier w/ the jcn under fwd bias. Likewise the base has an abundance of free holes, same reasoning.

It is impossible for an E field to selectively impart motion upon the emitter electrons & NOT on the base holes. How can Ib ever be zero? We can minimize Ib by doping base region very lightly w/ acceptors, much lower than emitter doping of donors. This reduces no. of holes injected into the emitter under forward bias, so that Ie >> Ib.

If we reduced base acceptor doping density to zero, we still have base current. Intrinsic silicon still conducts current in the presence of an E field. The device is now no longer n-p-n, but rather n-i-n ( "i" = "intrinsic").

To reduce Ib to zero literally means we must make the base an insulator. Then Ib = 0. There are 2 problems. The device is no longer "bipolar". The word "bipolar" literally means "2 polarities". An insulator for the base region makes the device unable to conduct electrons emitted from the emitter. When b-e is fwd biased, how can emitted electrons get through the base & onward to collector if said base is an insulator?

A device w/ Ib = 0 is simply NOT BIPOLAR. A fwd biased p-n b-e jcn in close proximity to a rev biased n-p c-b jcn is why the device is called bipolar. An insulated input electrode that forces Ib to zero is not bipolar.

We have a transistor constructed with an insulated input electrode such that the input current dc value is zero. It is a MOSFET. The JFET is similar only that the input jcn is reverse biased resulting in near zero gate current at low freq. Neither of these devices is classified as bipolar. Both are voltage controlled. They need gate current to operate. But Vgs is the quantity directly controlled, with Ig being indirect & incidental, yet important.

If a device has Ib = 0, it is not a bjt. It's that simple. How can a b-e p-n jcn be fwd biased w/o base current? Bill you have to examine what you're saying & do a sanity check. An insulated input device is not a bjt! Period.

How can I make things any clearer. If Ib were zero how do I explain bjt action? Well, if we make Ib very low, as in a superbeta device, Ie still controls Ic. My current control model, straight from OEMs, uses Ie to control Ic, not Ib. Ib is a factor, but Ie is what Drs. Ebers & Molls used in their 1954 paper as the control quantity for Ic. I have the paper if you need it. I'll email it to anyone.

Another question is "what if Vbe was zero?" An ideal p-n jcn under forward bias should have zero voltage at any finite current value. One could say that Vbe can vanish as well. If every electron emitted from emitter entered the p type base region w/o a potential barrier being formed, the case for a perfect rectifying p-n jcn, Vbe equal zero, but Ic is still alpha*Ie. This requires a material w/ a band gap energy of zero.

One can theorize zero Vbe and/or zero Ib. Real world semiconductors have non-zero Ib & Vbe both. If both vanished, we still have Ic = alpha*Ie. That is still the transistor action law. An emitter resistor is connected to the bjt, & input signal is at the base. Ideally zero current enters the base, & Vbe is zero. The entire input voltage appears across emitter resistor Re. Ic = -Ib + Ie = 0 + Ie = Ie. The output voltage of this emitter follower is exactly Vin.

Hence we have an ideal buffer. A voltage gain of exactly one (zero Vbe drop), w/ infinite input impedance (Ib = 0). Ideally Ib & Vbe can both vanish. But we always have Ic = alpha*Ie = 1*Ie.

In reality, like it or not, Vbe & Ib are facts we must deal with. I'd love it if my emitter followers could swing to the rail (Vbe = 0). I'd love to drive a 25 amp motor w/ zero base current in the bjt buffer.

I can wish for zero Vbe & Ib all I want. Wishing isn't getting.

Any other questions? Best regards.

Claude

 

[wbeaty]

If Ib was zero, it would NOT be a bjt!

OK. But first...

Do you now understand that my transistor article is not aimed at engineers? Right?

Did you take note of who the original intended audience was?

 

[wbeaty]

Any other questions?

Yes. Still looking for answers:

Do you now understand that my transistor article is not aimed at engineers?

When I stated who the original intended audience was, did you see that message?

 

[cabraham]

Yes. Still looking for answers:

Do you now understand that my transistor article is not aimed at engineers?

When I stated who the original intended audience was, did you see that message?

Yes, I am well aware of the target audience. Offering explanations that differ from established canon is not the problem. Bringing in info that counters the canon is where I have trouble.

I realize that QM, charge control, doping density, band gap, etc., is more than a lay person can digest. But you made statements that were downright contrary to canon. That is why I made my remarks.

Everybody has their unique way of explaining things, & I make no claims that my explanations are superior. Personally, I agree with your method of examining the device as 2 back to back diodes opposing. Also, it is all important that we distinguish between 2 diodes & an actual functioning bjt.

The best method I used to teach this concept to non-electrical majors (civil, machanical, chemical) when I taught at uni, was as follows. Make the base region so wide, say 1.0 mm, that every e- (electron) emitted recombines in the base region with an h+ (hole). Here we have 2 back to back diodes & nothing more. There is no transistor action whatsoever.

But there is a potential barrier, a depletion zone, & an exponential I-V curve.

Ie = Ies*exp((Vbe/Vt) - 1).

Ic = alpha*Ie = 0, since alpha is zero for such an arrangement.

We have no transistor action at all, nothing but a fwd biased b-e jcn diode, & a rev biased c-b jcn diode. The logarithmic/exponential relation holds steadfast because it is a diode relation. We still have no bjt functionality.

Reducing the base region width results in an increase in collector current Ic since alpha is now increasing. When the base region is very thin, on the order of 1.0 micron, Ic is almost the value of Ie.

In the 1st case we had near zero Ic, the 2nd case shows Ic almost equal to Ie. What changed? NOT the potential barrier relation, not the I-V "Ohm's Law" relation, not the Shockley diode eqn. What changed is alpha. When alpha is near zero, we have a pair of back to back diodes per the exponential eqn above.

Ie is exponentially related to Vbe, Ic = 0. In the 2nd case, alpha approaches 1 & Ic = alpha*Ie = alpha*Ies*exp( ).

Bill I would recommend approaching the explanation of bjt action when addressing lay people in terms of the base being too thin to allow recombination. I personally like your 2 diode approach. If you start w/ a p-n jcn diode, explaning recombination, you can then advance to bjt. The difference is that the e- from the emitter pass through the super thin base so quickly, there is not enough time for recombination. The current from base to emitter literally overshoots the base & gets yanked into the collector.

Anyway, I believe that something along those lines might work for lay people. Again, I don't claim to be the best teacher at all. I get your point about the target audience. Thanks for your feedback & interest. BR.

Claude

 

[wbeaty]

Yes, I am well aware of the target audience.

OK, thanks! And yes, comparing narrow base to extremely wide base is an excellent teaching strategy of which I previously hadn't heard.

Regarding canon, earlier you stated that the Ebers-Moll model is a CC model, and their original paper employed current-controlled current sources. Correct?

Are you certain that you're not misinterpreting something? N.b. that your assertion regarding Ebers-Moll being a CC model goes entirely contrary to numerous undergrad Uni books which give the following CV (large signal transconductance) equation as the central feature of Ebers-Moll model:

Ic=Is*(exp(Vbe/Vt)-1) (for large hfe of course, alpha ~=1)
​

Is this not canon? If the above CV equation isn't "Ebers-Moll model," then you've discovered a vast flaw in an enormous number of textbooks.

 

[Studiot]

Cabraham's approach in stressing the width of the interdepletion region is excellent and fundamental - or at least it was when I learned about transistors back in the 1960s. (post#68)

Pierce devotes several pages to calculations of the effect of base width on carrier density, potential diagrams etc and has an excellent diagram on page 101, which I have appended.

For expansion of these formulae (posts #68 & 69) I also recommend the monograph by E H Cooke-Yarborough of the Atomic energy Research Establishment at Harwell

An introduction to transistor circuits (1957)

go well

 

[cabraham]

OK, thanks! And yes, comparing narrow base to extremely wide base is an excellent teaching strategy of which I previously hadn't heard.

Regarding canon, earlier you stated that the Ebers-Moll model is a CC model, and their original paper employed current-controlled current sources. Correct?

Are you certain that you're not misinterpreting something? N.b. that your assertion regarding Ebers-Moll being a CC model goes entirely contrary to numerous undergrad Uni books which give the following CV (large signal transconductance) equation as the central feature of Ebers-Moll model:

Ic=Is*(exp(Vbe/Vt)-1) (for large hfe of course, alpha ~=1)
​

Is this not canon? If the above CV equation isn't "Ebers-Moll model," then you've discovered a vast flaw in an enormous number of textbooks.

Send me a message, & I'll email you the Ebers-Moll 1954 paper. It shows the bjt collector current being controlled by emitter current, i.e. a CCCS. Alpha is the parameter which determines the Ic value from Ie. They show a schematic w/ a CCCS controlling Ic.

The exp( ) relates the Vbe to Ic, but alpha is all important. The exp( ) covers the relation between Vbe & Ie. To get Ic we need alpha. The diode equation for the b-e jcn is Ie = Ies*exp((Vbe/Vt) - 1)> But Ic = alpha*Ie. So Ic = alpha*Ies*exp( ). The collector current is controlled by alpha*Ie. But Ie has a direct relation w/ Vbe as well as Ib.

All 3 eqns are relevant. A bjt offers both current gain as well as voltage gain. To compute current gain we use eqn 1) for the common emitter & emitter follower topologies. For the common base current gain, we use eqn 3). To compute voltage gain we use eqn 2). Again, all 3 eqns come into play when thoroughly analyzing bjt behavior.

We cannot make more than one quantity the controlling quantity. We generally control Ie, & Ic = alpha*Ie. Sometimes we control Ib, w/ Ic = beta*Ib. Usually this is not good due to beta dependency. We want networks that rely on well defined parameters like alpha & resistor values. Resistors have tight tolerances.

Send me a message, & I'll email you the E_M paper, the horses' mouth on the E-M eqns. BR.

Claude

 

[wbeaty]

I'm trying to understand the details of Ebers-Moll.

Based on your semiconductor expertise, would you say that the
Ebers-Moll model depicts the BJT as a current-controlled device?

Yes, that is what figure 5 on page 1765 depicts.

I'll take a look.

Beware, this could be a case similar to Maxwell's Equations. Go to Maxwell's original work and you won't find his four equations. Maxwell never actually wrote those four, and probably wasn't aware they existed. Maxwell attacked the problem in an obscure way, employing magnetic vector potential, quaternions, and writing twenty equations. Later scientists came in and revised everything, producing the four equations known today. If you rely on Maxwell as the "horse's mouth," then you'd be in big trouble. (Officially the four equations are today called the Hertz/Heaviside equations. But Maxwell discovered the original mathematical form which describes EM fields.) See 2008 Microwave Journal, 23 years: Acceptance of Maxwell Theory http://bit.ly/qRQNCH

 

[Studiot]

Beware, this could be a case similar to Maxwell's Equations. Go to Maxwell's original work and you won't find his four equations. Maxwell never actually wrote those four, and probably wasn't aware they existed. Maxwell attacked the problem in an obscure way, employing magnetic vector potential, quaternions, and writing twenty equations. Later scientists came in and revised everything, producing the four equations known today. If you rely on Maxwell as the "horse's mouth," then you'd be in big trouble. (Officially the four equations are today called the Hertz/Heaviside equations. But Maxwell discovered the original mathematical form which describes EM fields.)

Kirchoff's original statements are also quite different from what is often now presented under his name.
Maxwell's translation of K is quite interesting to read since if Professor Lewin followed the original he would not be able to present his famous 'conflict between K and Faraday lecture'.

This type of situation is actually not uncommon in the physical sciences.

 

[cabraham]

I'll take a look.

Beware, this could be a case similar to Maxwell's Equations. Go to Maxwell's original work and you won't find his four equations. Maxwell never actually wrote those four, and probably wasn't aware they existed. Maxwell attacked the problem in an obscure way, employing magnetic vector potential, quaternions, and writing twenty equations. Later scientists came in and revised everything, producing the four equations known today. If you rely on Maxwell as the "horse's mouth," then you'd be in big trouble. (Officially the four equations are today called the Hertz/Heaviside equations. But Maxwell discovered the original mathematical form which describes EM fields.) See 2008 Microwave Journal, 23 years: Acceptance of Maxwell Theory http://bit.ly/qRQNCH

So Bill, is this what you're saying? Your site claims that the CC model of bjt operation is wrong, & you are right. Then you use Ebers-Moll as the source for your claim that the bjt be classified as VC, not VC, because Ebers & Moll seem to suggest that. I just showed that the E-M paper published in 1954, depicts the bjt as CC, not VC. Now you're suggesting that Drs. E & M should not be viewed as the horses' mouth.

But they were your source for your thesis. My CC model is affirmed by every semicon OEM I know of, i.e. Tex Instr, Natl Semi, Fairchild, Intl Rectif, etc. Ebers-Moll is pretty accurate, but Gummell-Poon is an improvement. The G-P model includes an additional factor to account for Early influence. The Early voltage is denoted as "Va". So Gummell-Poon is as follows:

Ic = alpha*Ies*(exp((Vbe) -1)))*(1 + (Vce/Va)).

At large values of Vce, the collector current for a given Ib, or Vbe as well, is larger than that obtained at low values of Vce. I'm sure you are well aware of the Early effect, no need to elaborate. That is the main difference between E-M & G-P.

Again, let me re-iterate that we seem to agree that the current control bjt model is a good external model when internal physics need not be considered. But when the internal charge profile & device geometry is relevant, the EE canon uses charge control as the correct bjt model. You insisted on voltage control, relying on E-M as the source, which does not support such a claim.

So when we look inside the device, & we need a better model than current control, what do we use? I say charge control, as does every semicon OEM, & uni. I say "QC", & that is my final answer. Any questions? Anyone?

Claude

 

[wbeaty]

So Bill, is this what you're saying? Your site claims that the CC model of bjt operation is wrong, & you are right.

No, not exactly.

I still haven't stated my position. Such enormous messages, and all without knowing what my arguments are. I'm very confused.

I haven't clearly stated my reasoning yet, so do you want to hear it? That's my second question.

I could be wrong, but it looks to me like this is the problem here:

http://www.nizkor.org/features/fallacies/straw-man.html
Description of Straw Man
The Straw Man fallacy is committed when a person simply ignores a person's actual position and substitutes a distorted, exaggerated or misrepresented version of that position. This sort of "reasoning" has the following pattern: ...

​

I've been remaining silent on this issue because I'm waiting for you to notice that something important is missing. Also I'm waiting for you to stop repeatedly putting words in my mouth while constructing extensive counterarguments, over and over.

Looking back on this thread, it obviously hasn't worked.

So, what would work? You got me... I have no idea.

Suggestions from everyone would be welcome.

 

[cabraham]

Bill, please state exactly what you differ with in the EE canon. Your site makes some pointed criticisms regarding the way bjt operation is presented in engr colleges & semicon OEM app notes.

Where is the error in the common view of bjt operation? Where did I (Claude) err in presenting my understanding of bjt basics. Let's not be cryptic. Anything that I said that is not clear, or appears mistaken, or requires elaboration, is no problem at all AFAIC.

Again, I won't presume to know what your position is. So I'm asking you to state very plainly in detail the specific issues you have, if any, with OEM bjt models. We can take it from there. Best regards.

Claude

 

[wbeaty]

Bill, please state exactly what you differ with in the EE canon. Your site makes some pointed criticisms regarding the way bjt operation is presented in engr colleges & semicon OEM app notes.

I can't answer that without first giving background. My article is aimed at the general public, meaning ~13yr old kids. My goal was to explain simply the inner workings of bjts, and do it without relying on a single equation. Kids have no use for advanced models which cover high frequency operation, high power, etc.

Look, I couldn't explain resistors to the public if I was forced to include the complete high-freq LC model; all we really need is Ohm's law (although even Ohm's law is quite a bit too advanced.) Isn't that obvious? And I certainly wouldn't explain diodes to them using a charge control high-freq model. And Gummel-Poon CC model of the bjt is completely inappropriate for my audience.

That's why I keep asking, did you notice who my audience was? Yes Gummel-Poon obviously is required for VHF design, for accurate spice simulations, etc. But for explaining transistors to people with zero math skills, it's just ridiculous.

Is this clear? CC is wrong. It utterly fails. It's a complete mismatch for the task at hand. It's the wrong tool for the job. (If you're looking for a tool which always works in every situation, well, good luck with that.)

So, how would we answer the following question?

What's a good way to explain the inner workings of the BJT to the math-phobic general public?
​

Above is the whole point of my article.

 

[cabraham]

I can't answer that without first giving background. My article is aimed at the general public, meaning ~13yr old kids. My goal was to explain simply the inner workings of bjts, and do it without relying on a single equation. Kids have no use for advanced models which cover high frequency operation, high power, etc.

Look, I couldn't explain resistors to the public if I was forced to include the complete high-freq LC model; all we really need is Ohm's law (although even Ohm's law is quite a bit too advanced.) Isn't that obvious? And I certainly wouldn't explain diodes to them using a charge control high-freq model. And Gummel-Poon CC model of the bjt is completely inappropriate for my audience.

That's why I keep asking, did you notice who my audience was? Yes Gummel-Poon obviously is required for VHF design, for accurate spice simulations, etc. But for explaining transistors to people with zero math skills, it's just ridiculous.

Is this clear? CC is wrong. It utterly fails. It's a complete mismatch for the task at hand. It's the wrong tool for the job. (If you're looking for a tool which always works in every situation, well, good luck with that.)So, how would we answer the following question?

What's a good way to explain the inner workings of the BJT to the math-phobic general public?
​

Above is the whole point of my article.

I agree with that except for the part I highlighted in bold. So we agree that the discussion is limited to external mAcroscopic modeling, not internal physics. But then you state that CC "utterly fails", to which I say "say what!" The CC model is very accurate as long as we're dealing w/ speeds well below ft, & not saturating the device.

Of course there is no tool which works in every situation. I stated that repeatedly. Even QC (charge control) does not always provide a precise result. CC has its limitations, but since we're discussing external models, no internal physics, how is it that CC untterly fails?

Examine the following 2 current eqns:

1) Ic = beta*Ib.
3) Ic = alpha*Ie.

Again if "ft" is the device transition freq, & beta is the current gain, then fb (f_beta) = ft/beta. This "fb" value is where the beta value is 0.707 times the low frequency beta value. At freqencies below this value, eqns 1) & 3) are valid. You can rely on them provided the device is not used as a saturated switch.

The following eqn relates Vbe to Ic:

2) Ic = alpha*Ies*exp((Vbe/Vt) - 1).

Again, this is a perfectly valid relation at speeds below fb, & with the restriction that the bjt not enter saturation. These are the same restrictions for eqns 1) & 3).

For higher speeds and/or use of the device as a saturated switch, we must use QC. Stored minority carriers, distribution in space, reverse recovery charge, reverse recovery time etc. come into play. None of these useful parameters appear in eqns 1), 2), & 3).

Eqns 1), 2), & 3), are simply what I call the "terminal eqns". They are not a failure, just limited in scope. They are conditionally valid.

My final point, forgive me for repeating myself. Neither I nor the OEMs ever claimed that the CC model was valid for internal physics. We have presented the CC model as a superficial external estimate. It works under the conditions given above. No need to belabor the limitations of CC, as it is universally acknowledged.

For high speed operation, saturation, and internal physics analysis, the CC model is too oversimplified and cannot provide much help. But your whole point is to avoid complex math, & theory. On one hand you claim that CC fails, then you state that you wish to avoid heavy math & physics.

The eqn 2), Ic in terms of Vbe, is just as limited as are eqns 1) & 3). When the CC model is shown to be inadequate, you say we should use the VC model, I & the OEMs say QC model. That is where we differ. It's not about the limitations of CC model. We agree that CC is limited.

Our difference is in which model is more precise for the conditions where CC fails. Your view tha VC takes over has no support from any solid state physics theory. Only QC can handle the speed & saturation conditions. Neither Ic eqn, using Ib, Vbe, or Ie, can handle these conditions.

I'm at a loss to make it any clearer. Is my point clear?

Claude

 

[wbeaty]

I agree with that except for the part I highlighted in bold.

We've utterly failed to communicate.

I said this:

Yes Gummel-Poon obviously is required for VHF design, for accurate spice simulations, etc. But for explaining transistors to people with zero math skills, [ Gummel-Poon (CC) ] is just ridiculous. Is this clear? CC is wrong. It utterly fails. It's a complete mismatch for the task at hand. It's the wrong tool for the job.

I never said this:

CC is wrong. It utterly fails.

Please don't take my sentences out of context. That's close to being straw-man.

OK, one more time.

Do you understand who is the intended audience of my article? My article is for children and the general public. My transistor article is for children. As I said in my previous message, Gummel-Poon and CC is wrong for children. When explaining the inner workings of transistors to children, CC fails. Using it is just stupid. It's completely the wrong tool for children.. It's like using a screwdriver to hammer nails.

If I tried to use CC and some equations to explain the inner workings of the BJT, and my audience is children, then I'd be a failure.

So we agree that the discussion is limited to external mAcroscopic modeling, not internal physics.

No we do not. The goal of my article is to explain the inner workings of transistors to children.

The goal of my article is to explain the inner workings of transistors to children.

Perhaps I didn't explain this before?

:)

CC has its limitations, but since we're discussing external models, no internal physics

We are not discussing external models.

We are explaining the internal physics of transistors ...to children.
Again, how would we answer the following question?

What's a good way to explain the inner workings of the BJT to children?​

This question reveals the entire point of my transistor article.

 

[cabraham]

Bill, you ask "What's a good way to explain the inner workings of the BJT to children?"

My answer is, of course, the 2 diode back to back model. After diodes are explained to children, then the 2 diodes can be explained. A thick base region behaves like 2 back to back diodes & nothing more. As the base region is made thinner, transistor action is then observed.

The CC model, namely Ic = alpha*Ie, works very well. For 2 back to back diodes, the Ic value should only be a small value, that associated w/ the c-b jcn reverse leakage current. Namely Ic = -Ics*exp((Vbc/Vt) - 1). After all, the c-b jcn is reverse biased, so little current can flow.

The emitter current Ie is given by Ie = Ies*exp((Vbe/Vt) - 1), of course. So we have

Ic = -Ics*exp((Vbc/Vt) - 1).

Ie = Ies*exp((Vbe/Vt) - 1).

For 2 diodes, that is the truth, whole truth, & nothing but the truth.

But when the base is so thin that the 2 junctions are in extreme proximity, another term is added to the eqns.

Ic = alpha*Ies*exp((Vbe/Vt) - 1) - Ics*exp((Vbc/Vt) - 1).

The added term is in bold font. Personally, this explains bjt action w/o quantum mechanics. The "alpha term" accounts for the emitted carriers from the emitter, transporting right through the base before most can recombine, then continue onward into the collector. The better the transfer, the closer alpha is to unity.

Honestly Bill, if the target is children, & QM is off limits, is there a better explanation than the 2 back-back diodes w/ ultra-thin base width? If there is, please enlighten us. BR.

Claude

 

[wbeaty]

Bill, you ask "What's a good way to explain the inner workings of the BJT to children?"

My answer is, of course, the 2 diode back to back model.

A bit too sophisticated for this audience.

the Ic value should only be a small value, that associated w/ the c-b jcn reverse leakage current. Namely Ic = -Ics*exp((Vbc/Vt) - 1).

Way overboard, we need total, total simplicity. E.g. we'd assume this Ic ~=0.

After all, the c-b jcn is reverse biased, so little current can flow. The emitter current Ie is given by Ie = Ies*exp((Vbe/Vt) - 1), of course

No equations allowed. Again: the audience is children. Even Ohm's law is too much. To construct equation-free explanations, one first must learn to think in equation-free terms: pure concepts, and visual/verbal/intuitive language.

 

[wbeaty]

So, how can we explain BJTs to children? To design an explanation, first describe the basic BJT operation verbally:

1. Base current controls the BE junction voltage
2. BE junction voltage determines height of BE potential-barrier
3. That potential-barrier sets the rate of charges crossing the BE junction
4. Most carriers from the emitter make it all the way to the collector
5. Ic approximately equals Ie

No heresies so far? :)

 

[Studiot]

So, how can we explain BJTs to children?

I said before and I'll say again I like the picture of man operating a tap.

Children can easily understand how this can compass either polarity or FETs etc.

Otherwise you have all sorts of words they only half understand such as voltage, charge, current carrier etc.

(Are physicists really any better? )

 

[cabraham]

So, how can we explain BJTs to children? To design an explanation, first describe the basic BJT operation verbally:

1. Base current controls the BE junction voltage
2. BE junction voltage determines height of BE potential-barrier
3. That potential-barrier sets the rate of charges crossing the BE junction
4. Most carriers from the emitter make it all the way to the collector
5. Ic approximately equals Ie

No heresies so far? :)

I believe the problem lies in #3. How does the potential barrier "set the rate of charges crossing the b-e junction"? I'm afraid that we are right back at the endless chicken-egg riddle. Does voltage "set the current" or does "current set the voltage"? Until we grapple with that vexing riddle, we will argue ad infinitum, which I am not going to do with anybody.

The power/signal source driving the amp stage is what sets the current. Then when carriers cross the junction recombination takes place for a small minority of carriers. Ionization occurs, & the local E field increases slightly due to the slightly increased charge distribution at the barrier zone. Vbe increases slightly.

This results in a slightly greater Vbe drop at the increased current level. The important point is that the rate of charges crossing the junction is determined by the power incident on the signal source driving the whole network. Using my singing Susan example w/ a microphone works well. Sue imparts acoustic energy to the mic diaphragm. Mechanical/acoustic power is converted to electrical power. The voltage drops due to cable resistance, the rbb' base region resistance in the bjt, the Vbe drop, & re, all subtract from the mic generated voltage.

The Vbe drop does not solely determine Ie. Rather, Ie is the net voltage after drops, divided by the total loop impedance. Vbe does not control Ic, but they are intrinsically & indirectly related. Here's how the events take place.

1) Sue sings into the mic.
2) Current & voltage are generated at the mic diaphragm. The ratio of V to I is the mic cable characteristic impedance, Zo.
3) The I & V move along the cable & encounter the bjt amp stage. Along the way cable resistance results in collisions & a charge distribution forming a potential barrier. Signal gets attenuated.
4) At the amp stage input carriers incur base side collisions due to rbb', & emitter side collisions due to re. Attenuation occurs.
5) Then the carriers cross the b-e junction. Most nake it to the collector but a few recombine in the base & ionize local atoms.
6) This changes ther barrier charge density & potential. The current is again attenuated.
7) If Sue cranks up her volume, the additional charges outputted by the mic add to the b-e barrier increasing Vbe.

Bottom line, Sue is the prime mover, she makes everything happen. Her volume determines all currents & voltages. She can sing loud, soft, or in between. Nothing happens until Sue makes it happen.

Rbb', re, cable R, cable C, b-e diffusion capacitance, b-c Miller cap, & barrier potential play a role. But none of them exclusively determine the current crossing the junction. Sue determines that current mostly, but there are drops due to rbb, re, & Vbe that diminish Sue's output. If Sue's mic outputs Vmic, each quantity cable R, rbb, re, & Vbe drop a portion of Vmic, leaving maybe 0.78*Vmic, or 0.63*Vmic, whatever.

If all drops were zero. Sue dictates the current. But nonideal parameters mentioned above rob a portion of Sue's signal. Thus, Vbe, rbb, re, & others do indeed play a partial role in "setting the current", but all are minor roles.

Sue is tha main entity that "sets" & determines the current crossing the junction. Is my explanation clear. BR.

Claude

 

[wbeaty]

Once again again again: THIS IS FOR CHILDREN. Complexity is verboten, and complex-ifying a simple situation is not any sign of competence. "Complexifers" are extremely valuable as graduate textbook authors, as RF chip designers, and for writing the best SPICE models. But for talking to children, "simplifiers" are who we need to hire.

We simplify everything by removing every smaller effect we can think of: remove the AC-source (microphone) and inject a Base current, unless you prefer a DC voltage source. Assume that rbb and re is zero, remove high-power phenomena, use the magical zero-resistance cable found in all intro courses, treat the DC case alone while ignoring dynamic AC issues such as Miller and other parasitic capacitance/inductance. We want a clear view of the most important phenomenon, so we wipe away all smaller details from our window. We don't even mention these details. (If something forces us to do so, we can add some details back in afterwards.)

 

[wbeaty]

I believe the problem lies in #3. How does the potential barrier "set the rate of charges crossing the b-e junction"?

It's a diode.

:)

Is there really a huge controversy about how PN junctions actually work? Is there some thread on physicsforums with a long battle over explaining the diode?

And pay close attention to your question above: how does a potential barrier set the rate of charges crossing [a] junction? If textbooks are fairly unanimous regarding explanation of PN junction potential barrier and resulting currents, then they've answered your question.

 

[cabraham]

It's a diode.

:)

Is there really a huge controversy about how PN junctions actually work? Is there some thread on physicsforums with a long battle over explaining the diode?

And pay close attention to your question above: how does a potential barrier set the rate of charges crossing [a] junction? If textbooks are fairly unanimous regarding explanation of PN junction potential barrier and resulting currents, then they've answered your question.

Please elaborate. I don't want to be accused of building straw men, but is this what you're saying?

1a) Id = Is*exp((Vd/Vt) - 1), for a diode. Are you claiming what most contrarians claim that Vd "sets" Id? If so, I need to remind you that the same equation can be expressed as :

1b) Vd = Vt*ln((Id/Is) + 1).

Vd does not set Id. The 2 quantities are inclusive. A change in Id takes place ahead of the change in Vd. Vd does not determine Id. If you doubt me, I'd suggest setting up an experiment w/ a fast scope & observe which quantity changes first. A pulse generator plus a resistor is to be placed in series w/ the diode. A current probe & voltage probe are used to measure Id & Vd.

On his web site, contrarian Kevin Aylward bases his whole position on the "fact" that diode current is "controlled" by diode voltage. He refers to Shockley's diode equation, per 1a) above. I emailed him telling him that eqn 1b) is just as valid as 1a). He didn't respond.

Vd does not "set Id". Nor vice-versa. The power/signal source energizing the network along w/ device parameters are what "set the current".

Claude

 

[wbeaty]

Please elaborate. I don't want to be accused of building straw men, but is this what you're saying?

1a) Id = Is*exp((Vd/Vt) - 1), for a diode. Are you claiming what most contrarians claim that Vd "sets" Id?

Ah, so there *is* a controversy about diode explanations!

OK, then before anything else we need to get clear on some very basic physics. Not voltage sweeping charges out of silicon, that's controversial stuff apparently. Not resistor operation (too controversial?) Not the nature of conductors. Not charges accelerated by fields. All the way back...

Cut to the chase ...I'm "claiming" the same thing any intro physics text "claims" ...that e-fields cause charges to experience a force.

This simple basic fact is built into all of classical fields concepts: a gravity field applies a force to a point mass, a b-field applies a force to a magnet pole, an e-field applies a force to a point charge.

Do you object? Are these "contrarian?"

Conduction, resistors, diodes, transistors, all that stuff comes later. First let's get the above out of the way. Do you accept that, if we apply a uniform e-field to a region of space containing an electrically charged object, the object experiences a force?

 

[cabraham]

Of course F = q*E. But the E field is provided by an external source. The charges along the depletion layer do NOT "sweep" anything. Examine the polarity & it is clear that the local E field due to accumulated charges in the depletion region oppose charges from crossing the junction. When you say that E fields exert forces on charges, please be more specific. Which E field in which location due to which charges acting on which other charges.

You take a well known concept, such as F = q*E, then spin & interpret said concept in a contrarian manner. If your theory was valid, it would be what is taught everywhere. Sure E fields exert forces on charge. But remember that when an E field imparts force & energy to a charge carrier, it loses the same amount of energy & is replenished w/ displacement current from the exernal source maintaining said E field.

The depletion zone has a local E field which opposes the flow of charges across the junction. It is the external source of E field which makes conduction happen. You sound as if the local E field associated w/ Vbe is what exerts force on the charges. Is this your theory? The E field associated w/ Vbe opposes charge motion across the junction.

PLease explain how charges move through the silicon. Start w/ the external source, say a microphone. Then cover the charge motion through the mic cable, emitter & base regions, then motion across the junction. Thanks in advance.

Claude

 

[wbeaty]

OK, then before anything else we need to get clear on some very basic physics... I'm "claiming" the same thing any intro physics text "claims" ...that e-fields cause charges to experience a force... a gravity field applies a force to a point mass, a b-field applies a force to a magnet pole, an e-field applies a force to a point charge.

Do you object?

Of course F = q*E...

Sorry, I guess I wasn't clear enough. I think I'm close to our central disagreement. This is about clearly explaining electrical physics to the general public... and the concept that voltage causes current. But before examining silicon or even conductors, first the simplest basic situation:

E-fields cause charged objects to experience a force, F = q*E, but the reverse is not true: the force on that object is not the cause of the e-field.

Place an electron between the plates of a charged capacitor in vacuum, and the e-field between the plates will produce a force on the electron. But the force on the electron is not the cause of the e-field between the plates. In other words, in the equation F = q*E, the q*E causes the F, but the F does not cause the q*E.

Agreed?

If you find errors or unconventional concepts in the above, or see something I missed, please point it out.