BJT Operation

rspuzio

Joined Jan 19, 2009
77
> Or is this going to become a chicken-or-egg discussion?

Chicken-and-egg is exactly how I see this. My point being
that, in nature, quantities tend to come in pairs ---

chickens and eggs
position and momentum
angle and angular momentum
voltage and amperage
electric and magnetic fields
heat and temperature
intrinsic and extrinsic geometry of space
Steve B. and his wife ;) ---

in which the behavior of each element of the pair
depends on what the other is doing. In fact, there is
a wonderful physical theory, Hamiltonian mechanics,
which is based on such pairs and a mathematical
theory, symplectic geometry, which provides tools
for making such descriptions.

To me, arguing which of the two elements of such a
pair is more important seems about as silly as the
Lilliput egg debate because both are needed to support
each other. Rather, I think it is more fruitful to examine
how they depend upon each other.

> Imagine some physical process(es) where A causes B which
> causes C which causes D, D being some effect whose cause(s)
> we're investigating.

Ah, but what about when the processes are interdependent
so that A causes B and B causes A? What would you then say?
As my list is meant to suggest, such situations are quite
common in physics.

As for this issue of control, returning to your original
question, one can, in principle, control either kind of transistor
either way, but, in practice, not all the possibilities are
feasible, as we can see by considering the actual values
involved --- for simplicity I've picked round values in the
right ballpark.

FET

At pinchoff, the gate of my FET is at 1 volt relative to the source
and there is a charge of 1 picocoulomb stored in the gate.
Therefore, to pinch off my FET, I could either place a charge
of a picocoulomb on the gate, in which case it will adjust its
potential to 1 V or, alternatively, I could connect it to a 1 V
battery, in which case, 1 pC will flow to the gate.

However, 1 pC is an awfully tiny amount of charge, which
is why people doesn't go around trying to control FET's by
trying to control the charge, but rather by controlling the
voltage. The two exceptions I can think of are as follows:

1. When using an FET to measure tiny currents such as leakage
through an insulator. For instance, at a current of a picoampere,
it takes a second to fill up the gate capacitor.

2. When dealing with high frequencies, as pointed out by
Rob Jenkins earlier. For instance, 1 mA, 1 GHz A.C. consists
of around 1 pC of charge flowing back and forth.

The remaining 99% of the time, the charges involved are
way larger than a picocoulomb.

BJT

When the maximum current through the collector of my BJT
is 100 mA, there is 1.00 mA flowing in through the base and the
base is at 0.700 V relative to the emitter. Thus, to make 100 mA
flow, I could either push 1.00 mA through the base, in which case
it would adjust itself to 0.700 V or set its voltage to 0.700 V, in
which case 1.00 mA would flow through the base.

In this case, the quantities involved, 1 mA and 0.7 V, are quite
reasonable, but the rub comes when we consider changes in
them. In real life, nothing is perfect, which is why we have
tolerance bands on resistors. Suppose that our control quantity
is off by 1%. If the base current is 1.01 mA instead, then the
collector current will be 101 mA, which is 1% off. However, if
my base voltage is 0.707 V instead, then the collector current
is 132 mA, which is 32% off. Yeeouch! As if that isn't bad
enough, it only gets worse because the dependence of current
on voltage is exponential. For instance, if we are 2% high at
0.714 V, then the collector current becomes 175 mA. Thus,
in trying to use the base voltage as a handle on the collector
current we have about as much control as one would have
control over the scalpel in trying to perform surgery with oven
mitts on. Thus , pretty much the only time voltage control over
BJT's is used is in digital applications where one only needs to
turn the transistor on or off as opposed to analog applications
where the exact amount matters.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
Or is this going to become a chicken-or-egg discussion?
It already is (or was) in the thread from which this one branched off. The previous discussion essentially only dealt with two points of view--is a BJT current or voltage controlled (at the most fundamental level; for engineering purposes we all understand that either viewpoint is useful)? I point out that physical electronics texts consider that it is charge controlled, a third point of view.
 

beenthere

Joined Apr 20, 2004
15,819
Can we separate charge from voltage? Can we separate current from voltage? Can we consider current without voltage or charge?

Perhaps you can posit an ideal universe where voltage can exist without an imbalance of charge, and charges can move without the prompt of voltage. That lets us consider the effects of charge and voltage as self-contained entities on ideal devices.

That universe will not resemble our world very usefully, though.

You wish to discuss a device as being charge controlled. Perhaps this is an ideal device in a different universe.

Can you explain how the charge accumulates without the influence of voltage, and how the charge moves without current? Is this a quantum tunneling charge? If so, how is the quantity of charge controlled?
 

steveb

Joined Jul 3, 2008
2,436
:

A transistor is a charge-controlled device.
I've thought about mentioning this in past discussions, but was afraid that if the voltage vs. current debate was so lively, the voltage vs. current vs. charge debate might get uncontrollable. :) Hopefully, that won't happen now.

You are probably well-aware that the Gummel-Poon transistor model is sometimes called "charge-controlled" or "integral-charge-controlled". Charge certainly plays an important role.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
Can we separate charge from voltage? Can we separate current from voltage? Can we consider current without voltage or charge?
I don't think it's necessary that, for example, current be an effect that can exist in isolation from voltage to allow me to say that current causes voltage, or that voltage causes current.

Suppose we have "thing", A, which always causes another "thing", B. And suppose that only A causes B; no other "thing" can cause B.

Since A causes B (by hypothesis), then it must be that whenever A is present, B will be present, and whenever B is present, A will be present.

The fact alone that the two are always present together doesn't allow us to determine the direction of causation.

But, neither does it allow us to say that causation doesn't occur in only one direction.

We will have to determine causation by consideration of more than mere association.

You can see that a question arises of just what do we mean when we say A causes B. If A and B always occur together, one is certainly led to think that there is a causative link. Often, a delay in the appearance of A or B after B or A is first detected can provide the necessary information to determine the direction of causation (other than mere association).

And, of course, if two "things" always occur together, both could be caused by a third "thing". Mere association just doesn't suffice to determine whether causation goes from one of the two to the other, or not.
 
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Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
I've thought about mentioning this in past discussions, but was afraid that if the voltage vs. current debate was so lively, the voltage vs. current vs. charge debate might get uncontrollable. :) Hopefully, that won't happen now.

You are probably well-aware that the Gummel-Poon transistor model is sometimes called "charge-controlled" or "integral-charge-controlled". Charge certainly plays an important role.
So of the three, current, voltage and charge, which do you see as the more fundamental "cause" of collector current?

It seems to me that charge is the more fundamental entity. Charge is physical. Voltage is an effect of charge unbalance, and current is a rate of change; neither are physical substances.

By the way, check your email.

It was probably your request that prompted me to bring up the charge control point of view.
 
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beenthere

Joined Apr 20, 2004
15,819
As voltage can be defined as imbalance of charge, it makes your statement -
We will have to determine causation by consideration of more than mere association.
a philosophical discourse on the chicken and its association with and possible primacy over the egg.

How does one determine if the voltage increased and caused charge to move into further imbalance, or did some cause add/subtract and electron and thus change the voltage? It might be difficult to determine if there was any delay at all in the increase in charge imbalance and the change in voltage, or vice versa.

I'm not trying to gnaw your ankle bloody - I'm curious why it might be important to consider a physical entity as if the real world manifestations of that entity could be disassociated from it.
 

steveb

Joined Jul 3, 2008
2,436
So of the three, current, voltage and charge, which do you see as the more fundamental "cause" of collector current?
I don't want to shy away from a direct question like this, especially since you were good enough to use the word "cause" rather than "control".

If forced to answer, I guess I would say charge. But really, my brain is not big enough to wrap around this issue. I see it as a circle. A specific charge relates to a specific voltage, even with the voltage/charge dependent capacitance. The establishment of charge requires the continuous current flow in the BJT, and the transient current flow in the MOSFET. Somehow a charge seems more real to me. Voltage is energy per unit charge, so voltage is more nebulous, because it does not tell you that a charge is present. It only tells you the energy that the charge will have if it is present. If enough charge is present, it will change the voltage. Also, a charge, with no external electric field (i.e. voltage gradiant) to put force on it, has no energy, so the surrounding device physics involving energy bands infuenced by voltage and charge is an intimate part of all this. So the decision between charge and voltage is difficult for me. I can justify to myself to exclude current as a direct cause, and prefer to think of it as a useful indirect control in both the BJT and MOSFET, but don't ask me to prove it because I don't know how.


By the way, thanks for tracking down the Gummel-Poon paper for me. I would have had to wait till after labor day to get to my University Library, and then, who knows if they would even have it. I appreciate it very much!!!
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
As voltage can be defined as imbalance of charge, it makes your statement - a philosophical discourse on the chicken and its association with and possible primacy over the egg.

How does one determine if the voltage increased and caused charge to move into further imbalance, or did some cause add/subtract and electron and thus change the voltage? It might be difficult to determine if there was any delay at all in the increase in charge imbalance and the change in voltage, or vice versa.

I'm not trying to gnaw your ankle bloody - I'm curious why it might be important to consider a physical entity as if the real world manifestations of that entity could be disassociated from it.
From what I read of modern physics, it may be possible for subatomic particles to simply appear from nothing out there in interstellar space, or to disappear. I think protons can disappear.

Anyway, I can imagine that an electron (or proton) might just suddenly appear where there was none before; nor was there any electric field (not much anyway). As soon as the electron appears, an electric field will begin to spread out at the speed of light. I find this scenario more believable than that an electric field might suddenly appear. If an electric field did suddenly appear, would its new existence cause the creation of an electron (or other charged particle)?

It seems more reasonable to me that the sudden appearance of a material object could then cause a non-material "thing" to appear, than to suppose that the sudden appearance of a non-material "thing" could then cause the creation of matter.

But, then I don't know anything about the sudden appearance of electrons; only what I've read.
 

beenthere

Joined Apr 20, 2004
15,819
Anyway, I can imagine that an electron (or proton) might just suddenly appear where there was none before
That scenario does pop up. It's a bit like the old Fred Hoyle theory of continuous creation, where new matter just popped out of the vacuum more or less at random. (Someone ran the numbers and found it brought about the end of the universe rather sooner than later, though.)

So far, so good. Let's see how this relates to charge in MOSFET's and later in BJT"s.
 

beenthere

Joined Apr 20, 2004
15,819
Anyway, I can imagine that an electron (or proton) might just suddenly appear where there was none before
That scenario does pop up. It's a bit like the old Fred Hoyle theory of continuous creation, where new matter just popped out of the vacuum more or less at random. (Someone ran the numbers and found it brought about the end of the universe rather sooner than later, though.)

So far, so good. Let's see how this relates to charge in MOSFET's and later in BJT"s.
 

rspuzio

Joined Jan 19, 2009
77
From what I read of modern physics, it may be possible for subatomic particles to simply appear from nothing out there in interstellar space, or to disappear. I think protons can disappear.
Hang on there --- while particles can appear and disappear, charge
conservation holds in modern physics just as much as in the physics
of Maxwell's day. When a charged particle appears, there must also
appear a particle of the opposite charge.

Neither quantum mechanics nor relativity theory invalidates what
beenthere said about charge and voltage going together. Dragging
in tunneling, wormholes, or any other exotic scenario won't make
a fundamental difference, only complicate the analysis at best.

From my experience studying, teaching and working with quantum
mechanics and relativity theory, I would say that modern physics has
really driven home the point that one can't dissociate the real world
manifestations from physical quantities. Already in Newton's day, Berkeley
criticized him for introducing absolute quantities and, a century later, Mach
refined the critique. Until about 1900, this philosophical issue could be
brushed aside, but when examining atomic and smaller scales as well as
cosmological scales, one arrives at a point where sloppy thinking
on this point is no longer possible. When I look at various paradoxes
and the like intended to show that there is something wrong with
modern physics, what I find is that, as soon as I think carefully about
physical quantities in terms of their manifestations and take care to
use operational definitions, the problems go away --- maybe what modern
physics predicts sounds odd, but it is not illogical. Also, whenever
someone has made a prediction based on assuming physical quanitities
independent of manifestation, experiment shows the prediction wrong.
(I am thinking particularly of the Aspect experiments here.)

Alright, maybe this is going too far and opening some other cans of
worms, but, since quantum statistical mechanics is used to explain how
semiconductors work, maybe it is not completely tangential to the
main agrument.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
Hang on there --- while particles can appear and disappear, charge conservation holds in modern physics just as much as in the physics of Maxwell's day. When a charged particle appears, there must also
appear a particle of the opposite charge.
Is there a rule that says the particle of opposite charge has to appear within, say, 1 micron of the first?

Why can't the particle of opposite charge appear 1 light year away? That way charge conservation holds for the universe.

And, the effect around the one particle is that it can't know about the other for 1 year.

Like I said, I don't know the details of all this, but I feel much more comfortable with the idea of a particle (mass) suddenly appearing and then causing an expanding electric field, than I do with the notion of an electric field suddenly appearing and creating some mass to go along with it.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
To me, arguing which of the two elements of such a
pair is more important seems about as silly as the
Lilliput egg debate because both are needed to support
each other. Rather, I think it is more fruitful to examine
how they depend upon each other.
How electric fields depend on charge distribution has been well examined in the past, and is well understood. There's not much more to say about that relationship.

So we discuss the not-so-well understood aspects.

> Imagine some physical process(es) where A causes B which
> causes C which causes D, D being some effect whose cause(s)
> we're investigating.

Ah, but what about when the processes are interdependent
so that A causes B and B causes A? What would you then say?
As my list is meant to suggest, such situations are quite
common in physics.
How can we know that "...A causes B and B causes A". I submit that just because they are always associated with each other we can't know if:

1. A causes B.
2. B causes A.
3. A causes B, and B causes A simultaneously.
4. something else causes them both simultaneously.

The mere fact that they always occur together doesn't suffice to distinguish the cases.

And, the fact that we can't distinguish the cases doesn't mean that one of them can't be true. Without other information we can't tell. As I said earlier, it calls into question the very concept of causation.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
So far, so good. Let's see how this relates to charge in MOSFET's and later in BJT"s.
Does your saying this mean that you're waiting for me to make the connection?

I think I already made it when I said that I think charge is the final cause of collector current and drain current, rather than voltage.

Giving my reasons for thinking charge causes voltage rather than the other way around is intended to strengthen the notion that charge is the final cause of collector current and drain current.

If in fact voltage is "caused" by unbalanced charge, then to me that settles it; it's charge rather than voltage that "causes" collector current.

I say "to me", because I know that I can't prove that charge "causes" voltage and not the other way around. If it were capable of proof, we wouldn't be discussing it at such length; we would just give the proof. I can only give the reasons that sway me, and I find the fact that charge seems to be matter (or part of matter) persuasive.

Electric fields are simply a way of describing the force that a test charge feels in empty space because of an arrangement of material objects (charges) elsewhere. I find it more satisfying to think that particular property of a region of space is not causative, but that the arrangement of charges is.

And if the electric field causes a force to act upon a test charge, I subscribe to an action-at-a-distance notion that says it's really the arrangement of charges at one place that causes a force to be felt elsewhere, the electric field being a convenient fiction to describe things which we can describe but really can't explain; we can only describe. So, to me, it's the charges that are causative.

You may feel differently.

My feelings on the matter of causation are weak, anyway. We don't really know what's going on; it just feels better to think we do.

But, I think we've got the what happens part down fairly well even if we still have a ways to go on the why and how part.
 

beenthere

Joined Apr 20, 2004
15,819
This is the chicken or egg problem -
If in fact voltage is "caused" by unbalanced charge, then to me that settles it; it's charge rather than voltage that "causes" collector current.
Unless it can be demonstrated that one can exist without the other, the argument goes nowhere.

As far as my feelings go, I see no way to persuade charge to move into a PN junction or into a gate structure without voltage.

Is voltage the control mechanism? No, it is no more than a force that moves charge into a location and at a concentration to enable a field to do work, whether by forming a conduction channel, or influencing the motion of electrons and holes in the substrate. Charge is the physical agent responsible, but can't get there or do work without voltage (and current).

I can't see one existing without the others (voltage, charge, current).
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,971
This is the chicken or egg problem -

Unless it can be demonstrated that one can exist without the other, the argument goes nowhere.
If when you say the argument goes nowhere you mean that it provides no proof that charge causes voltage; I acknowledged that. But, I don't agree that it's because it can't be demonstrated that one can exist without the other.

As I've said, the mere fact that they always exist together doesn't prove that one doesn't cause the other, and it doesn't prove that one does cause the other, either. It proves nothing about causation. It's a necessary, but not sufficient, condition for causation to exist.

Two "things" in a causal relationship such that A always causes B, and only A causes B, will, as a logical consequence, always occur together (if they occur at all), but the association alone can't be used to prove or disprove the causal relationship.

There might be other ways to prove that one causes the other without requiring a demonstration of the existence of one without the other. In fact, there would have to be another way if you want proof, bacause as I point out in the previous paragraph, if there is that type of one-way causal relationship between two "things", they will always occur together; it won't be possible to have one without the other. Searching for a demonstration of one without the other will fail because it's a logical impossibility. But the inability to find such a demonstration doesn't prove that there is no causal relationship.

However, to us humans, when two "things" always occur together, our brains are wired to infer causation. This tendency gets us into trouble sometimes, but often we're right.

Is voltage the control mechanism? No, it is no more than a force that moves charge into a location and at a concentration to enable a field to do work, whether by forming a conduction channel, or influencing the motion of electrons and holes in the substrate. Charge is the physical agent responsible, but can't get there or do work without voltage (and current).

I can't see one existing without the others (voltage, charge, current).
A "...force that moves charge into a location..." sure sounds like a "control mechanism" to me. Although, as steveb points out, using the word "control" rather than "cause" may be a different issue.

I also can't (at least as the universe appears to be constituted) see one existing without the others, but that doesn't prove that one can't be the cause of another.

When you say "Charge is the physical agent responsible", that sounds to me like you're getting very close to acknowledging that charge "causes" the drain current.

I spoke about being more comfortable with some charge springing into existence and then causing an electric field, rather than a field springing into existence and then causing some matter to exist.

I intended that to be a plausibility argument, not proof, that charge causes electric fields. All of what I've said has been in the nature of plausibility argument. I don't know how to prove it, and I haven't yet seen any disproof in this thread, only unplausibility arguments.
 

beenthere

Joined Apr 20, 2004
15,819
I am happier saying something more like the conventional - that the electric field opens (or closes) the conduction channel, thus promoting - or pinching off - conduction. Voltage is the published agent, but charge does seem to be the other side of that coin.

I do not separate voltage and charge, because one must involve the other. That may be a further discussion to identify or reach some definition of "control" and "cause".

I won't readily imagine how some quantum effect can cause charge to appear on a gate at just the time one wishes the device to operate.
 
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