# How Transistor works - verifying

by Nikarus
Tags: transistor, verifying
P: 963
 Quote by wbeaty Just two questions. First, might you yet recall which text you yourself had in your undergrad semiconductor physics course, the one that first covered transistors? Second question: I haven't clearly stated my reasoning yet, so do you want to hear it? That's my second question.
Kittel - solid state physics as undergrad.
Muller & Kamens - Device Fabrication For Integrated Electronics at grad school, MSEE.
Sze - Physics of Semiconductor Devices, grad school Ph.D.-EE.

If you have reasoning you'd like to explain, sure, by all means do so.

Claude
P: 963
 Quote by Studiot Many thanks for this answer. I have now had time to review this website with the following results. 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'. 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'. There was significant discussion, including the nature of the term 'control', though not of the gargantuan proportions of the one in 2010 in cabraham's link. I apologise to claude if we had primed the pump for that argument in 2008. I felt that his summary in post#39 here was particularly good. However I would take issue with equations posted in post#50. My version of equation (3) has another term which is significant in certain types of transistor and reminds us that there are other agents that affect, and therefore can 'control', the collector current. 3) Ic = alpha*Ie + Ico go well
Thanks for your feedback. I am well aware of the additional "Ico" term in eqn 3), & what you've presented is correct. But when we describe quantities that "control" a device, we are not usually referring to leakage & other parasitic flaws.

In a bjt, the objective is to control collector current w/ some sort of input signal, as found in uctlrs, transducers, photodiodes, etc. The inherent leakage current Ico, which exists due to the non-ideal nature of the reverse biased c-b jcn, is present & varies greatly w/ temp.

Of course Ico plays a role in determining Ic, but it is not something we use to control Ic. It is an inherent property of the bjt, one which fortunately has a very small influence on bjt Ic behavior, for silicon material.

In the 1950's when germanium was the dominant bjt material, Ico was a real problem at medium to high temps. Designers had to account for the large Ico c-b leakage when employing Ge devices at temps above 50 or 75 C. The limit was around 100 C.

Then silicon replaced Ge around 1959, & Ico for Si is generally small enough to neglect. Again, it's there, but Si devices can operate to the mil temp range of 125 C & beyond w/o Ico being too large an eror. It is an error term for sure, & your eqn is more precise than the simplified version I presented.

But Ico, & I believe I'll get universal backing/concensus on this, is NOT a "control" quantity. It influences Ic for sure, but we don't control Ic by setting a value for Ico. I think this whole question revolves around the meaning of "control".

Ib, Vbe, Ie, Vbc, Ies, Vt, Ico, etc., all have influence over Ic depending how the device is driven. If the b-e jcn is driven by a true current source or voltage source w/ a large series resistor, then Vt, & Ies determine the Vbe value at a given temp. Also, Ico adds to whatever Ic value is obtained from eqn 1) or 3).

Which quantity are we adjusting to get a specific value of Ic? That is what we mean by control. Again, your version of eqn 3) is more precise than my simplified version. Ico exists indeed & influences Ic. But if Ico is 2.7 uA, & we bias the bjt at Ic value of 1.0 mA, the error is just 0.27%. For larger Ic value, the error is less.

Again, w/ Ge devices, the presence of large Ico values forced the designer to take it into consideration. Circuit topology was built around the need to mitigate large Ico values. EEs from the 1950's can give you insight into this practice. BR.

Claude
P: 5,462
 I think this whole question revolves around the meaning of "control".
I have no doubt you have great knowledge of semiconductors and have already commented how worthwhile your technical analyses are.

But I also feel there is a lack of acceptance that others may also have something valid to say and a corresponding willingness to listen to them as well as to expound to them.

Here is a simple experiment to prove that it is possible to 'control' Ic with the base not even connected.

You can show this by allowing photons to enter the transistor, biased suitably between collector and emitter only. It is possible to switch on the transistor by this means.

As a matter of interest how do multiple emitter transistors fit into your control scheme? and how do you describe control by emitter injection?
P: 963
 Quote by Studiot I have no doubt you have great knowledge of semiconductors and have already commented how worthwhile your technical analyses are. But I also feel there is a lack of acceptance that others may also have something valid to say and a corresponding willingness to listen to them as well as to expound to them. Here is a simple experiment to prove that it is possible to 'control' Ic with the base not even connected. You can show this by allowing photons to enter the transistor, biased suitably between collector and emitter only. It is possible to switch on the transistor by this means. As a matter of interest how do multiple emitter transistors fit into your control scheme? and how do you describe control by emitter injection?
Thanks again Studiot for your feedback. With photon stimulation, instead of forward biasing the base-emitter junction, photons provide the energy to transition valence electrons into conduction. Though the base lead is not brought out to the outside world, the base-emitter junction carries a conduction current due to photon stimulation.

There is no base lead terminal, but there is a base region internally. Emitter region emits electrons which are yanked into the collector via the E field of the reverse biased c-b jcn. Have I answered your question.

Your last question is a good one. "Control by emitter injection" is how I describe the bjt in general. Emitter electrons injected towards the base quickly become collector current. Base holes injected into the emitter recombine there w/ electrons in the emitter. But base injection does not directly produce collector current.

The reason the base is doped w/ acceptor ions (for a p type base, i.e. npn bjt device), is to improve c-b reverse breakdown voltage & leakage current. A heavy doping of acceptor atoms into the p base results in more base current for a given collector curret, an undesirable thing. But doing so reduces Ico, a bad thing, & improves c-b junction blocking voltage ability.

What function does the injection component of base current serve is as follows. THe lower the doping density in the base, the lower the base injection current & the higher the beta value. Superbeta bjt's at op amp inputs use these devices. But the c-b blocking voltage ability is a few volts, & the leakage current from c-b is horrendous.

For bjt devices that are to operate w/ a Vce of 50, 100, or more volts, w/ low leakage current c-b, i.e. Ico, the base must be doped heavier than is optimum for high beta. Thus current gain is sacrificed for low Ico & high c-b blocking voltage.

It's all about tradeoffs. Ic is produced by Ie. Ib is needed, but we wish to minimize it. If we minimize Ib too much, Ico goes through the roof, & Vce,blocking plummets. So base doping is optimized according to how high Vce must block, & how low Ico needs to be. Higher voltage devices have lower beta. Can't get around that. Did I help?

Claude
P: 5,462
 Thanks again Studiot for your feedback.
When I look over this thread I see many of the same points and formulae that were raised in the 2008 thread I linked to.

In particular, since others may have missed them

1)What happens if you try to force current control via the base.

2)What happens if you force voltage control via the base.

3)What happens at different frequencies - a very important point since the bjt is not only a DC device.

4)Whether you are interested in the internal workings of the bjt or worings of a circuit using a bjt.

5)The role of the emitter.

The 2008 thread I linked to contains some further information, including references to orifinal articles and other practical demonstrations on how to vary Ic including by varying the input frequency only (= control of Ic by frequency) and a great deal more on the photon aspect.

I would agree the demonstrations were contrived to show that any one method only achieves partial control, since it can be subverted by another.
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.

Thank you for some excellent insights.
P: 116
 Quote by cabraham Kittel - solid state physics as undergrad. Muller & Kamens - Device Fabrication For Integrated Electronics at grad school, MSEE. Sze - Physics of Semiconductor Devices, grad school Ph.D.-EE.
Thanks! I wanted to make sure we were "on the same page," approximately. My own text titles I don't quite recall, but I think they're in a cellar box from 1979. I'm almost certain that the main one was Sze above.

 Quote by cabraham If you have reasoning you'd like to explain, sure, by all means do so.
OK. But first...

Do you now understand that my article is not aimed at engineers? Right? Did you take note of who the original intended audience was?
P: 5,462
 Did you take note of who the original intended audience was?
That too is a very good point.
P: 116
 Quote by Studiot 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.

 Quote by Studiot 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: Simplifers and complexifiers

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.
P: 116
 Quote by Studiot 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? :)
 P: 5,462 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
P: 963
 Quote by wbeaty 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
P: 116
 Quote by cabraham 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?
P: 116
 Quote by cabraham Any other questions?

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?
P: 963
 Quote by wbeaty 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
P: 116
 Quote by cabraham 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.
 P: 5,462 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 Attached Thumbnails
P: 963
 Quote by wbeaty 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
P: 116
Quote by cabraham
 Quote by 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

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