Transistor theory question involving the base current

[Osnel Jr]

TL;DR Summary

I know that the base to emitter current is what makes the transistor essentially turn on but why doesn't that deplete the second barrier and only the first

Help

 

[DaveE]

I'm not sure I understand your question. For an NPN BJT in normal (linear) operation, the BE junction is forward biased and the CB junction is reverse biased with an associated depletion region. The base current doesn't deplete anything, it allows electrons to enter the depletion region and generate more current across the CB junction.

One (not very precise) way of thinking about it is some of the electrons that flow from the emitter end up in the depletion region, because the base region is thin, and are swept to the collector by the strong e-field in the depletion region.

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/bipolar-junction-transistors/#:~:text=The%20bipolar%20junction%20transistor%20(BJT,holes%20in%20the%20same%20crystal.&text=The%20bipolar%20junction%20transistor%20shown,and%20a%20base%20in%20between.

 

[LvW]

Do not overestimate the role of the base current.
(In this context, one of the worlds best known D&D engineers - the late Barrie Gilbert - has used the term "defect" and "nuisance" for the base current).
Without any doubt , there is always a base current (which can be accounted for during the design of BJT-circuits), however, it is only the base-emitter voltage Vbe which determines the collector current (Shockleys famous equation).
This can be (and was) demonstrated and prooved.
(There is not a single proof or explanation for current control)

 

[cabraham]

Do not overestimate the role of the base current.
(In this context, one of the worlds best known D&D engineers - the late Barrie Gilbert - has used the term "defect" and "nuisance" for the base current).
Without any doubt , there is always a base current (which can be accounted for during the design of BJT-circuits), however, it is only the base-emitter voltage Vbe which determines the collector current (Shockleys famous equation).
This can be (and was) demonstrated and prooved.
(There is not a single proof or explanation for current control)

Barrie Gilbert had little formal tech education beyond high school. He went far with what little he knew but his explanations are sheer nonsense.
Nuisance - can be applied to Vbe as well. Germanium bjt had Vbe of 0.2-0.3 volts. Silicon goes 0.6-0.7. This limits the swing of an emitter follower driving a load.
"Defect"? A p type base & n type emitter are forward biased by an external source. Emitter electrons move towards base & collector. Base holes move towards emitter.
These base holes comprise nearly all the base current at low frequency. A forward biased p-n junction exhibits forward current. No defect here.
Gilbert said many times that base current should be zero, but how can a forward biased diode have zero current. Ideally Vbe should be zero since a forward diode ideally has no voltage drop.
Current control model was published in the Ebers-Moll 1954 paper. Emitter current controls collector current per
Ic = alpha*Ie.
Shockley equation relates p-n junction current & voltage, Id = Is*(exp(Vd/Vt)+1)
Combining gives Ic = Ies*(exp(Vbe/Vt)+1).
Ie & Vbe are related via Schockley. But Ic = alpha*Ie, is the equation for transistor action.
As electrons cross the base into collector, while holes cross from base to emitter, depletion zones are firmed resulting in barrier potential. This barrier is nearly equal to Vbe, & is determined by the current. A lab test affirms that Ie changes ahead of Vbe.

 

[LvW]

Hi Claude...You will remember some of our earlier discussions on this matter.
I agree that - as you wrote - emitter current controls collector current per Ic = alpha*Ie.
No doubt about this.
And therefore (as you have stated): Ic = Ies*(exp(Vbe/Vt)+1).

Final conclusion: Vbe determines (controls) Ic.
The base current Ib plays no major role - as far as the voltage gain of amplifier stages is concerned. The existence of Ib just reduces the input resistance of BJT based circuits.
(As THIS was the original question).
Do you agree?

(By the way: According to my knowledge, Barrie Gilbert has "never said many times that base current should be zero"). His position was that Ib does not "control" (determine) Ic in a cause-effect-sense. And therefore, this current would be more or less parasitic.)

 

[cabraham]

Hi Claude...You will remember some of our earlier discussions on this matter.
I agree that - as you wrote - emitter current controls collector current per Ic = alpha*Ie.
No doubt about this.
And therefore (as you have stated): Ic = Ies*(exp(Vbe/Vt)+1).

Final conclusion: Vbe determines (controls) Ic.
The base current Ib plays no major role - as far as the voltage gain of amplifier stages is concerned. The existence of Ib just reduces the input resistance of BJT based circuits.
(As THIS was the original question).
Do you agree?

(By the way: According to my knowledge, Barrie Gilbert has "never said many times that base current should be zero"). His position was that Ib does not "control" (determine) Ic in a cause-effect-sense. And therefore, this current would be more or less parasitic.)

Parasitic? That means it does nothing. Base current can be useful.
Emitter follower - load is on emitter side. Load current equals Ie. But Ie = Ib + Ic. So Ic & I are powering the load. Nothing parasitic here.
But the input source is Vg. An emitter follower ideally buffers a voltage source by presenting high input Z & low output Z.
The output, Ve (emitter voltage wrt ground) is one forward drop below Vg, ie Ve = Vg - Vbe. In this anecdotal case, Vbe does not power the load, & Vbe is a fraction of the input source Vg, that does not reach the output load.
In this case, Ib powers the load, Vbe is a loss, or parasite.
Common emitter - the load is on the collector side, powered by Ic & Vcc, the rail supply. Hence one can view Ib as a parasite, since it doesn't power the load. But the same can be said for Vbe. Both can be called parasitic.

Common base - same as common emitter.

Ib can be viewed as parasitic as it does not power the load in 2 topologies, CE & CB. In CC (EF) topology, Ib powers load.
Vbe never drives a load. It is always a fraction of input voltage lost due to forward drop of b-e junction.

In addition, even when Ib does not power load, it's existence is an inevitable result of base region doping during fabrication.
A low density of acceptor ions diffused into base results in high beta, low Ib. But tradeoff is low Vce breakdown voltage & high collector base leakage current. These are undesirable.
A bjt with low Vce breakdown rating will *punch through* if Vcemax is exceeded. The Ib value is nice & low, as we wish. But collector to emitter punch through can be catastrophic.
So kit's increase the doping density of acceptor ions, ie boron, in the base region during fab. The result is a bjt with lower collector to base leakage current, higher Vcemax, & less prone to punch through. The tradeoff is higher Ib. Not something we want, but increased Ib is worth it to avoid punch through, no debate there.
Vbe is not useful in & of itself, but it is a byproduct of fabrication to avoid bigger problems. Early bjt was made with germanium. This material has lower forward voltage drop, but higher reverse leakage current.
The Ge parts collector to base reverse leakage was bad at temps as low as 100 C. When you were turned off, they were not fully off.
1959 - Silicon arrives. Si parts have much lower reverse leakage current. Ge diodes & bjt had high reverse leakage, but low forward drops.
So diodes & bjt have low reverse leakage with higher forward drops. Tradeoff just like the Ib case.
Why use Si when Ge offers lower drop? The crossover distortion in class B amps is a problem. Instead of 0.7 volt drop, lower is better.
We use Si material & accept 0.7 volts, not because more Vbe is good, but because less reverse leakage is great. Vbe, like Ib, is a quantity we wish to minimize.

The "current driven" model if the bjt refers to the EMITTER current, NOT BASE current.
Vbe & Ib are not that useful, except in emitter follower where Ib powers load.

Questions welcome.

 

[cabraham]

Anyway, the ebers-moll model published in 1954 shows bjt as Ic = alpha*Ie, not

beta*Ib. The i-v diode files are a simulation. Signal generator voltage is stepped into resistor & diode. The diode current is a quick staircase waveform. The diode voltage is slower, the current hits full value as the voltage hardly budged. Diode voltage is slow but settles to final value Ling after current settled. Next jump repeats the process.
Shockley: 2 forms of equation
1) Id = Is*(exp(Vd/Vt)-1)
2) Vd = Vt*ln((Id/Is)+1)

Many blindly assume that the voltage determines or "controls" the current. But the simulation shows that voltage on a p-n junction changes as a result of a current change. The current goes from existing value to the new value quickly. Voltage slowly catches up.
Diode current is NOT "controlled" by diode voltage.

 

[DaveE]

Is it voltage or current controlled? Isn't that like asking is the heat in a resistor caused by it's voltage or current? You have both; you always have both. If you have a model that you like and that works, use that one.

I also think talking about "gain" depends greatly on the application. In some circuits voltage gain makes sense, in others current gain is easier to use.

 

[Baluncore]

Many blindly assume that the voltage determines or "controls" the current. But the simulation shows that voltage on a p-n junction changes as a result of a current change. The current goes from existing value to the new value quickly. Voltage slowly catches up.

At last we have narrowed it down to two possible answers to the question of which came first, the chicken or the egg. We can simulate a chicken laying an egg, or we can simulate an egg hatching into a chicken. A simulation can show anything we want, depending on the order and direction of computation.

 

[LvW]

At last we have narrowed it down to two possible answers to the question of which came first, the chicken or the egg. We can simulate a chicken laying an egg, or we can simulate an egg hatching into a chicken. A simulation can show anything we want, depending on the order and direction of computation.

Yes - I fully agree. Simulation is mathematics, nothing else. Our simulation programs can never reveal if the effect A is the cause or the result of the effect B. These simulations are based on relations only (forward/backwards).
However, concerning the question voltage/current, I think it is clear: Current is always the result of a voltage. No current (in our electronic devices) without a driving E-field (caused by a voltage).

 

[LvW]

Parasitic? That means it does nothing. Base current can be useful...
Vbe is a loss, or parasite....
Hence one can view Ib as a parasite, since it doesn't power the load. But the same can be said for Vbe. Both can be called parasitic.

OK - perhaps I have used a term ("parasitic") which is not correct...or it must be defined before.
To me - "parasitic" means: A property which is not intended and which has an unwanted influence on the desired circuit behaviour (like the parasitic capacitance of a circuit node).
In this sense - the voltage Vbe is certainly NOT a parasitic one because it is a voltage which is externally applied (and not the result of an unwanted effect).

 

[LvW]

Many blindly assume that the voltage determines or "controls" the current. But the simulation shows that voltage on a p-n junction changes as a result of a current change. The current goes from existing value to the new value quickly. Voltage slowly catches up.
Diode current is NOT "controlled" by diode voltage.

I do not intend to place a comment to this surprising claim.
Just one remark: Are we allowed to "blindly" trust simulation results ?
I doubt if such a simulation can correctly reproduce the time relationships between physical processes (cause and effect), unless they are intentionally included in the models.

 

[cabraham]

At last we have narrowed it down to two possible answers to the question of which came first, the chicken or the egg. We can simulate a chicken laying an egg, or we can simulate an egg hatching into a chicken. A simulation can show anything we want, depending on the order and direction of computation.

Very good point! The l-V relation pretty much is chickens & eggs. I've said that for years.

 

[cabraham]

OK - perhaps I have used a term ("parasitic") which is not correct...or it must be defined before.
To me - "parasitic" means: A property which is not intended and which has an unwanted influence on the desired circuit behaviour (like the parasitic capacitance of a circuit node).
In this sense - the voltage Vbe is certainly NOT a parasitic one because it is a voltage which is externally applied (and not the result of an unwanted effect).

But Ib is externally applied as well. The external source powers resistors & the b-e junction. Vbe is a drop incurred by charges moving through the n & p material. You claim that Vbe is externally applied, which is only true if b-e is shorted directly across a voltage source, which is certain doom for the bjt.

 

[LvW]

But Ib is externally applied as well. The external source powers resistors & the b-e junction. Vbe is a drop incurred by charges moving through the n & p material. You claim that Vbe is externally applied, which is only true if b-e is shorted directly across a voltage source, which is certain doom for the bjt.

"...charges moving..."
May I simply ask - which force causes the charges to move?

 

[cabraham]

"...charges moving..."
May I simply ask - which force causes the charges to move?

The signal generator. Could be an antenna, phone cartridge, microphone, etc.
For example, Sue sings into a mic, charges are moved by her vocal energy. If mic is dynamic, her air pressure from her voice moves the mic diaphragm & charges are moved. This wave travels through the mic cable & reaches the bjt in the mic preamp. The charges move through base & emitter regions, crossing junction & forming depletion zone barrier.
My Sim showed I-V relation with diode. Charges move through anode & cathode, resulting in forward voltage drop.

Sue the singer causes charges to move right at the mic diaphragm. The charges travel through the cable reaching bjt b-e junction. The depletion barrier potential has not changed yet, since charges have not crossed barrier.
Ib & Ie have changed due to Sue singing. These charges cross the barrier changing depletion barrier. The barrier potential cannot change without charge moving through base & emitter regions.

Vbe very nearly equals built in potential Vbi. Sue vocal pressure moves charges from mic to b-e junction. Vbe changes due to Sue, as does Ib & Ie. But Vbi changes as a consequence if charges crossing junction & recombinant. Then Vbi changes, & incoming charges feel an opposition due to the local E field in the depletion region.

Sue caused all this to happen. Ib & Ie are not caused by Vbe. They are all caused by Sue. Ib & Ie change BEFORE barrier potential has the chance to change.
The idea that Ib/Ie change as a *consequence* of Vbe changing is pure prejudice. Shockley equation is steady state, not transient.

After the depletion zone changes due to Sue, & things settle, the Ib-Vbe-Ie relation per Shockley is on effect. Momentarily during transients, the I-V relation does not follow Shockley.

 

[LvW]

Thank you for the long and very detailed answer.
However, with all respect - I did not want to learn something about Sue ...I only wanted to clarify which electronic force causes the charges to move.
To my opinion, it is an E-field within a conducting material. Perhaps I am wrong and it is the "vocal energy of Sue" ? A hard discussion...
(Sorry...but I could not resist to be ironic).

 

[cabraham]

And how are E fields set up inside conducting material?
By transporting charges, of course. If Sue stays silent, how can the bjt have E fields created inside its b-e region?

E fields are the result of the external mic, after being energized by Sue's vocal energy. Charges are transported the cable to the bjt. When charges cross b-e junction, electrons from emitter cross base region, where they then enter collector. Emitter electrons mostly survive the trip thru base, a few get recombined with base holes. The great number that reach the collector have high mobility. The collector is n type so electrons are majority carriers inside collector & emitter as well, being n type.
But the few electrons that recombine in base with holes form one edge of the b-e depletion zone. The electrons are MINORITY carriers when they are inside the p type base region.

Holes from base move towards emitter, but once they enter emitter, n type region, the holes are a MINORITY carrier, & recombine quickly near the edge of the emitter. This is the other end of the depletion zone.

Thus an E field is formed by these minority carriers. This E field integrated over the depletion zone width forms the Vbe. Before Sue stimulated the mic, Vbe was at its quiescent value determined by bias network.

in order to stimulate bjt charges from outside world enter b-e region. These charges in motion are Ie & Ib. When the charges reach the edges of emitter & base regions Ie & Ib have been perturbed from their quiescent value.
A moment later after charges crossing depletion zone & recombinant, Vbe gets perturbed.

Ib, Ie, & Vbe all change as a result of mic action sending charge carriers towards bjt.
Ie & Ib change immediately when new charges enter b-e regions. Vbe changes a bit later due to junction crossing & recombination.
I'm at a loss to make it clearer.

A bjt is not easy to understand. The 3 dimensional charge motion, diffusion, recombination, etc., is not trivial. Even brilliant minds need time & effort to understand it.

Even when I was age 30, I was not fully aware of all this. It took me years to find out all of this. Even a very smart person will struggle at first with this stuff I reviewed.

 

[Baluncore]

Looking at it differently, the base drive energy arrives along a bundle of Poynting vectors, that reach the BE junction as an EM wave guided between the two conductor (probably asymmetric) transmission line. Let us assume that the impedance of the line is matched to the BE zone of the transistor.

As the electric field appears across the junction, the magnetic field guiding current will flow over and begin to diffuse into the BE junction that terminates the line.
What is it about the BE junction? Do the charge carriers react to the transverse electric field, to the magnetic field, or to the skin surface current?
Do PNP and NPN behave differently? Is one electric and the other magnetic?

 

[LvW]

And how are E fields set up inside conducting material?
By transporting charges, of course.

OK - when it is not the E-field (caused by the applied voltage) which is responsible for the movement of charges (because you say that the E-field is "set up" by the moved charges), I must ask again ( as in my post#15):
Which force causes the charges to move?

 

[cabraham]

OK - when it is not the E-field (caused by the applied voltage) which is responsible for the movement of charges (because you say that the E-field is "set up" by the moved charges), I must ask again ( as in my post#15):
Which force causes the charges to move?

Electric force does that. Singer Sue outputs acoustic energy that mic transduces into E field. This sets charges in motion. Charges move through mic cable & reach bjt b-e junction. Ib, Vbe, & Ie get changed from these new charges showing up. Any elaboration still needed.

 

[LvW]

Electric force does that. Singer Sue outputs acoustic energy that mic transduces into E field.

OK - from the beginning, this was my conviction, of course.
But in post#14 you wrote: "Vbe is a drop incurred by charges moving through the n & p material."
To me, this is a kind of contradiction.
Either Vbe is a drop - caused by moved charges; or it provides the electric force (E-field) which causes the charges to move.

 

[anorlunda]

Either Vbe is a drop - caused by moved charges; or it provides the electric force (E-field) which causes the charges to move.

I think you are trying too hard to apply cause and effect, first comes X then Y. The relationship V=I/R is circular. (Even when R is nonlinear and non-constant, it's still circular.)

In a circular relationship, there is no first and last. Voltage causes the current and current causes voltage. Resistance (constant or not constant) merely describes the relationship between V and I. You can turn that around by saying V and I define R, so we can't even say that R comes first.

Once again, it is circular. Therefore, there is no either or as you said.

 

[LvW]

I think you are trying too hard to apply cause and effect, first comes X then Y. Voltage causes the current and current causes voltage.

Sorry, I cannot agree to that.
Current causes voltage?

Lets assume we look at a simple resistor.
When the current through this resistor can produce the voltage across this device - which force drives resp. allows the current ? Current is identical to movement of charges - without an E-field there can be no continuous flow of charges.
(I speak about the common understanding of the quantity "current" within an electrical circuit resp. within parts like resistors or semiconductors; I do not refer to movement of charges caused by a chemical process or by diffusion/drift effects.)

Of course, during calculations and analysis of electrical circuits we can ASSUME that according to V=I*R the current I could "produce" the voltage V - from the math point of view this works.

However, asking which comes first (as a cause of the effect) it is always the voltage which comes "first". No current without a driving E-field within a conducting material.

 

[anorlunda]

However, asking which comes first (as a cause of the effect) it is always the voltage which comes "first". No current without a driving E-field within a conducting material.

You said that before. It's not true. Consider an inductor L with an initial current I0. Now we switch the inductor to disconnect it from the charging circuit, and put in in parallel with a resistance R or a Diode D. From the R's view, a current suddenly appeared which causes a voltage to appear across the R or D device.

The inductor is just a current source. There are many other kinds of current sources. Consider a simple solar cell. Depending on the operating point, it can act like a current source (I versus V nearly horizontal) or like a voltage source (I versus V nearly vertical).

 

[LvW]

Consider a simple solar cell. Depending on the operating point, it can act like a current source (I versus V nearly horizontal) or like a voltage source (I versus V nearly vertical).

As I have mentioned already before : Having such a discussion in a written form is problematic and can cause misunderstandings.
You say:..."can act as a current source or voltage source"...certainly true when we are using the terminology of our dayly life (labor jargon).
In this sense - also a battery can "act" as a "current source" when we have a sufficiently large source resistor. However, the primary source of electrical energy is a voltage, correct?
And this question is - as far as I want to understand the discussion - the core of the problem.

The same applies to the solar cell mentioned by you. It provides an "open source" voltage of app 0.5 V - and as you know, it is usual practice to connect more such cells in series for producing a larger voltage. Of course, it can deliver a current when there is a closed current loop - but without such a loop?
Hence, the primary energy source in the solar cell is a voltage

In physics, I think, every effect has its cause (source). Do you think that the pair voltage/current is the only exception to this rule?

(PS: From tomorrow I will be traveling for 10 days...so I cannot continue this discussion, sorry).

 

[anorlunda]

In physics, I think, every effect has its cause (source). Do you think that the pair voltage/current is the only exception to this rule?

Believe me, you don't want to dig down to the fundamental physics of electricity which is QED. But if you're talking circuit analysis, which I believe we are, then the relationship is circular. Circuit analysis is compatible with physics as long as certain assumptions are valid. You can call it either simplified physics or computational tricks that are not physics; the difference is semantic.

I won't engage in a meaningless semantic debate. If you insist that every current source is really a voltage source go ahead. But your either-or is still false.

Either Vbe is a drop - caused by moved charges; or it provides the electric force (E-field) which causes the charges to move.

 

[LvW]

I won't engage in a meaningless semantic debate. If you insist that every current source is really a voltage source go ahead. But your either-or is still false.

Is it - in your eyes - really a "semantic debate" when we discuss the nature of an electrical energy source?
Do you realize how this discussion has started? Somebody was interested to learn something about the role of the base current - and we arrived very soon at the related problem if the collector current is determined by the base current or the base-emitter voltage.
In some contributions I have learned that (a) the musical power of Sue is responsible and (b) the E-field will move the charged carriers thereby creating the E-field.
It is my only intention to solve this contradiction - and you are speaking of "semantics". Surprising.

 

[cabraham]

Sorry, I cannot agree to that.
Current causes voltage?

Lets assume we look at a simple resistor.
When the current through this resistor can produce the voltage across this device - which force drives resp. allows the current ? Current is identical to movement of charges - without an E-field there can be no continuous flow of charges.
(I speak about the common understanding of the quantity "current" within an electrical circuit resp. within parts like resistors or semiconductors; I do not refer to movement of charges caused by a chemical process or by diffusion/drift effects.)

Of course, during calculations and analysis of electrical circuits we can ASSUME that according to V=I*R the current I could "produce" the voltage V - from the math point of view this works.

However, asking which comes first (as a cause of the effect) it is always the voltage which comes "first". No current without a driving E-field within a conducting material.

But the "driving E field" is your assumption. How does a battery produce an E filed & voltage. The chemical process, redox, propels positive ions towards positive terminal, & negative ions towards neg terminal.
In this case, this current moves against the E field & increases it. Inside the battery currents move opposite to E field.
A current can give rise to an E field, just as E field can motivate current.
"CURRENT" is merely moving charges. But don't forget that charges are all surrounded by their own E fields. An electron unattached sitting in space is surrounded by its own E field.
E fields are not caused by voltage! How can you think that. E fields are constructs used to define interaction between charges.
Voltage is defined as work done against or with E field. Voltage itself has no meaning without E field. Voltage does not produce E field. A charge has its E field. Another charge has its E field. If I move 1 charge towards or away from the other, energy has changed. The energy change divided by the charge is the voltage change.
No cause & effect here. E fields are inherent in all charged bodies.
Currents consist of charges in motion. When current enters a resistor, these charges collide with lattice ions in resistor material. An electronic can fall in energy level from conduction to valence band, adding a charge of -1. This charge has its E field. The integral of E filed over distane is voltage. A current, ie charges in motion, entered a resistor. The moving charges collide with lattice ions, charges fall down from conduction to valence, changing the charge, E field & voltage.
A current passion g through a resistor caused a voltage drop.

 

[eq1]

This thread kind went off topic but the answer to the original question is: charge doesn't move from C to B because that would be uphill in energy when the NPN is in active mode. [1] Why is the collector at the lowest energy? Because that's how the NPN is constructed. We dope the collector that way on purpose.

I know textbooks like to draw BJTs like back to back diodes but a BJT is not a back to back diode. (I suppose it is that, but it is also more.) If you want proof try the following experiment. Wire two diodes back to back on a bread board, see if you get a transistor. When you understand why that didn't work you'll understand BJTs.

Personally, I like to think of the BJT as voltage controlled. V(B,E) modules the height of the barrier from E-B. When the barrier is low enough charge from E can flow into B and then it's all downhill from there. Personally, I am not a fan of the water analogies but it's kinda like the lowering of a flood gate in a dam, once the gate gets below the water line, things start to move. In a dam the force moving the water is gravity. In a BJT its diffusion moving the charge. And it kinda shows how the amplification works, i.e. a small movement of the gate height leads to a large change in the amount of water in the river below. And I suppose it can even have some implications on this discussion. i.e. for the dam, which controls the movement of water? The flood gate or gravity? And of course, for the case of the dam, the answer is both. But the analogy is getting a bit stretched.

My $0.02 anyway.

[1] https://en.wikipedia.org/wiki/File:Bjt_forward_active_bands.svg

 

[cabraham]

Believe me, you don't want to dig down to the fundamental physics of electricity which is QED. But if you're talking circuit analysis, which I believe we are, then the relationship is circular. Circuit analysis is compatible with physics as long as certain assumptions are valid. You can call it either simplified physics or computational tricks that are not physics; the difference is semantic.

I won't engage in a meaningless semantic debate. If you insist that every current source is really a voltage source go ahead. But your either-or is still false.

This thread kind went off topic but the answer to the original question is: charge doesn't move from C to B because that would be uphill in energy when the NPN is in active mode. [1] Why is the collector at the lowest energy? Because that's how the NPN is constructed. We dope the collector that way on purpose.

I know textbooks like to draw BJTs like back to back diodes but a BJT is not a back to back diode. (I suppose it is that, but it is also more.) If you want proof try the following experiment. Wire two diodes back to back on a bread board, see if you get a transistor. When you understand why that didn't work you'll understand BJTs.

Personally, I like to think of the BJT as voltage controlled. V(B,E) modules the height of the barrier from E-B. When the barrier is low enough charge from E can flow into B and then it's all downhill from there. Personally, I am not a fan of the water analogies but it's kinda like the lowering of a flood gate in a dam, once the gate gets below the water line, things start to move. In a dam the force moving the water is gravity. In a BJT its diffusion moving the charge. And it kinda shows how the amplification works, i.e. a small movement of the gate height leads to a large change in the amount of water in the river below. And I suppose it can even have some implications on this discussion. i.e. for the dam, which controls the movement of water? The flood gate or gravity? And of course, for the case of the dam, the answer is both. But the analogy is getting a bit stretched.

My $0.02 anyway.

[1] https://en.wikipedia.org/wiki/File:Bjt_forward_active_bands.svg

Vbe does NOT "modulate height of barrier". Also how can diffusion "move the charges"? Diffusion is the tendency for charges to spread out until all regions have equal concentration.
The barrier in a bjt without being energized is the thermal voltage, roughly 25.9 mV, usually rounded off to 26 mV.
What modulates this barrier is changes in current. Vbe changes after current has changed. Say the bjt is biased at Ic = 1.0 mA with Vbe = 0.65 V. What changes barrier value?
At 1.0 mA, charges cross junction & recombine. Vbe stays at 0.65 V since recombination rate & transport rate are in equilibrium.
The external source, Sue the singer, starts singing, which moves charges from mic, through cable, to bjt. This increase in charge results in more recombination in the depletion zone, raising the barrier slightly. Equilibrium occurs when the barrier increases to balance recombination with increased charge density.
The current increased due to Sue energizing more charges. The extra charges transported through b-e junction resulted in more charges accumulating in the depletion zone, most reach the collector resulting in increased Ic. Some recombine, increasing the recombined charges in depletion zone, increasing Vbe.
Look at a bjt data sheet & a graph is usually given for Vbe as a function of Ic or Ie.
Ie & Ib changed as a result of Sue. Vbe changes later due to increased recombination in base-emitter depletion zone.
Vbe does not force Ie to change. It does not change barrier value. Vbe changes as a result of barrier being subjected to a change in charge transport.
Semiconductor physics texts will affirm this.

 

[eq1]

Also how can diffusion "move the charges"? Diffusion is the tendency for charges to spread out until all regions have equal concentration.

In your mind, what is the difference between "move" and "spread out"?

You definitely seem curious about the topic but I think you have quite a lot of misunderstandings. I can highly recommend Chenming Hu's (TSMC Distinguished Professor Emeritus and 2020 IEEE Medal of Honor winner) Semiconductor Textbook, which should also qualify as a physics textbook. It was written with the practitioner in mind so it might be more up your alley than the more conventional physics textbooks.

I think there are many sections of it you would find interesting but I'll point out this particular section of chapter 8 as it seems to be on topic.

https://www.chu.berkeley.edu/modern...-integrated-circuits-chenming-calvin-hu-2010/

"Figure 8–1b shows that when the base–emitter junction is forward biased, electrons are injected into the more lightly doped base. They diffuse across the base to the reverse-biased base–collector junction (edge of the depletion layer) and get swept into the collector. This produces a collector current, IC. IC is independent of VCB as long as VCB is a reverse bias (or a small forward bias, as explained in Section 8.6). Rather, IC is determined by the rate of electron injection from the emitter into the base, i.e., determined by VBE. You may recall from the PN diode theory that the rate of injection is proportional to e^(qVBE ⁄ kT) . These facts are obvious in Fig. 8–1c."

 

[cabraham]

Ic is NOT "determined" by Vbe. Ic changes as a result of Ie changing. Vbe catches up eventually.
Many references, including uni texts, often use the Shockley equation
Ic = alpha*Ies*exp((Vbe/Vt)-1) as "proof" that currents Ie or Ic are "controlled" or determined by Vbe. But Shockley's equation is just as easily expressed as follows:
Vbe = Vt*ln((Ie/Ies)+1).
You & some text authors insist that changes in Ie/Ic are a *result* of changes in Vbe. A lab bench & scope test are needed.
I submitted a simulation showing diode voltage as changing after diode current has changed. LvW insisted that this Sim is only as accurate as the model used. Maybe lab set up is nevessary.

 

[eq1]

But Shockley's equation is just as easily expressed as follows:
Vbe = Vt*ln((Ie/Ies)+1).

You’re wielding Shockley’s equation like it’s an axiom but it’s not. It’s a solution to a very particular system known as a BJT. But, where did Shockley derive it from? And what experimental evidence supports those equations? And what do they have to say about this topic? I would write it down but Dr. Hu has done an excellent job for me and I doubt I can improve upon his work. In addition Berkeley has made his work available for free! I suggest you take advantage of it.

Edit: I also just remembered Sedra & Smith has a pretty decent derivation of the Shockley equation (and MOSFET and PN junction for that matter) just using dimensional analysis which might be more approachable. It's definitely not free but it's been a standard microelectronics text for decades so I think most libraries, pretty much anywhere in the world, should be able to obtain it. The problem is that derivation will only work for a 1d transistor, basically one line in the cross section. But it illustrates the point nicely and succinctly.

 

[cabraham]

You’re wielding Shockley’s equation like it’s an axiom but it’s not. It’s a solution to a very particular system known as a BJT. But, where did Shockley derive it from? And what experimental evidence supports those equations? And what do they have to say about this topic? I would write it down but Dr. Hu has done an excellent job for me and I doubt I can improve upon his work. In addition Berkeley has made his work available for free! I suggest you take advantage of it.

Edit: I also just remembered Sedra & Smith has a pretty decent derivation of the Shockley equation (and MOSFET and PN junction for that matter) just using dimensional analysis which might be more approachable. It's definitely not free but it's been a standard microelectronics text for decades so I think most libraries, pretty much anywhere in the world, should be able to obtain it. The problem is that derivation will only work for a 1d transistor, basically one line in the cross section. But it illustrates the point nicely and succinctly.

I realize SE (Shockley equation) is NOT an axiom. In semiconductor physics class,cwe derived it as homework.
It is not a solution to bjt, but solution to diode. The SE describes the math relation between diode voltage & current in forward direction, or to limited extent in reverse before Zener breakdown occurs.
A bjt b-e junction is a diode & thus described by SE. But SE gives the relation between Ie & Vbe, or Ib & Vbe.
Transistor action is as follows:
Ic = alpha*Ie. This is the TAE, transistor action equation.
Thus when we combine SE with TAE, we get
Ic = alpha*Ies*(exp(Vbe/Vt)-1).
Read my earlier post where a p-n junction diode was connected to a staircase generator & resistor, & observe the I & V diode waveforms. It is clear that current settles before voltage.
The current in a diode determines the voltage, not the other way around.
As soon as the generator increases its voltage its current increases. The charges move towards the diode. When they arrive, anode & cathode current both increase, but it takes time for Vd, forward voltage drop of diode, to settle to its final value.
LvW insisted that simulator models are suspect, & that Sim results are only as reliable as the models, I've witnessed such waveforms in labs on a scope.
A lab test will affirm that p-n junction current changes ahead of the junction voltage.