Transistors

[Ratch]

Hi Ratch
I must say what a very good explanation of how a BJT works . I am a bit confused well it don't take much. I was once told in a post that if I wanted to be pendantic then a BJT was in fact a field effect device. I was not sure but ended up using this in a presentation amongst other technical people at work. They looked at me. Bit confused I must say. But thinking I had been educated on here I felt quite confident.It appears they might have been wrong.

If whomever told you that meant a BJT and a FET worked the same way, he was dead wrong. A BJT is a bipolar device. It uses two different charge carriers (holes AND electrons) to make a current. A FET is a unipolar device. It uses one charge carrier (holes OR electrons) to make a current. A BJT controls the collector current by sending it THROUGH or across the base-emitter depletion region that is controlled by the Vbe voltage. A FET controls the drain current by sending it ALONG or parallel to a depletion region formed into a "channel", whose widening or narrowing is controlled by the Vgs voltage.

If this is the case then I am pissed to be honest.
Now this person might have been on about the electric field that is developed between the BE junction that pushes the electrons into the bases neutral region. I am not sure.
So I am now confused about the whole electric field thing.

I keep telling you that it is diffusion that brings the charge carriers into the base region, not Vbe. What Vbe does is lower the barrier voltage so that more charge carriers can diffuse into the base region and be swept into the collector by the voltage difference between the base and collector

I thought that if you had a potential difference you had an electric field between the two so more charge on one side than the other. And in a transistor say the collector is higher porential than the emmiter you have an elctric field between the two so why don,t you have one across the base emmiter juntion which pushes electrons into the base. Then does this not match what Steve is saying.

Your statements and question are not very coherent. I don't know if you are describing a capacitdor or a PN junction. Again, charge carriers are not pushed or pulled from the emitter into the base by Vbe. They are moved by diffusion.

If not can you explain the difference.
I understand that the BJT is a diffusion device and it does not need an electric field to do so. But it does this because of the difference of charge between the emitter and collector. But if this charge difference is a potential difference why is there not an electric field.
Thanks
Adam

What can I say? Diffusion results from the concentration difference of holes and electrons of a PN junction. The P side has an abundance of holes, the N side has an abundance of electrons. They get together on the PN boundary and annihilate each other to form a depletion region.

Ratch

 

[Laplace]

...
1) Models show what a device does, not how it works. The makeup of the model can have no relationship with the physics of the device.
...
Ratch

From the standpoint of a circuit designer what is most important? Is it the circuit model or the device physics? I get the fact that Ratch likes to discuss device physics, but look at a small-signal BJT model such as the hybrid-pi and you will see only resistors, capacitors, and a current source. There are no PN junctions, depletion region, majority & minority carriers, or diffusion process in the BJT circuit model. You might ask how anyone can use a device without understanding how it works? Simply by using the circuit model which is composed of circuit elements. At best, focusing on device physics will just confuse the design & analysis process. I would like to say I'm willing to be convinced otherwise, but I have spent a very long time successfully ignoring device physics.

 

[Ratch]

From the standpoint of a circuit designer what is most important? Is it the circuit model or the device physics?

For designing circuits, one wants something that helps make a device do something. One doesn't really care how it works. So one concentrates on the device model.

I get the fact that Ratch likes to discuss device physics, but look at a small-signal BJT model such as the hybrid-pi and you will see only resistors, capacitors, and a current source. There are no PN junctions, depletion region, majority & minority carriers, or diffusion process in the BJT circuit model.

Perhaps so, but one will be a better designer if the device physics can be correlated with the model. The number of models are numerous, because they are only good for a specific circumstance and frequency range. Notice how the models change at high frequency, but the device physics do not.

You might ask how anyone can use a device without understanding how it works?

The same way lots of folks operate a machine or drive a car without knowing how it works. Another question is why do the model users tread into device physics by saying that a BJT is current controlled, and that Ib controls Ic. They should instead acknowledge that a BJT is voltage controlled, but aver that a current model of a BJT is useful.

Simply by using the circuit model which is composed of circuit elements.

Someone had to devise the model in the first place. The model was based on knowledge of how the device worked.

At best, focusing on device physics will just confuse the design & analysis process. I would like to say I'm willing to be convinced otherwise, but I have spent a very long time successfully ignoring device physics.

I certainly never said that one should not use models in circuit design. But, knowing how a device works certainly helps. Winfield Hill's method of voltage-centric design came about because of his understanding of device physics.

Ratch

 

[Laplace]

I agree that someone has to devise the circuit model in the first place, and this person needs to be really smart about how the device works so that the device physics correlate well with the model. But why do the model users tread into device physics by saying that a BJT is current controlled, and that Ib controls Ic? Perhaps they are referring to the circuit model rather than the device physics. Haven't you ever personified a model as the real thing?

 

[Ratch]

I agree that someone has to devise the circuit model in the first place, and this person needs to be really smart about how the device works so that the device physics correlate well with the model. But why do the model users tread into device physics by saying that a BJT is current controlled, and that Ib controls Ic? Perhaps they are referring to the circuit model rather than the device physics. Haven't you ever personified a model as the real thing?

No, I have not. I like to keep myself in the real world. The following link shows what is typical from hundreds of sources of false information in textbooks and lectures.

Ratch

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html

 

[Arouse1973]

If whomever told you that meant a BJT and a FET worked the same way, he was dead wrong. A BJT is a bipolar device. It uses two different charge carriers (holes AND electrons) to make a current. A FET is a unipolar device. It uses one charge carrier (holes OR electrons) to make a current. A BJT controls the collector current by sending it THROUGH or across the base-emitter depletion region that is controlled by the Vbe voltage. A FET controls the drain current by sending it ALONG or parallel to a depletion region formed into a "channel", whose widening or narrowing is controlled by the Vgs voltage.




I keep telling you that it is diffusion that brings the charge carriers into the base region, not Vbe. What Vbe does is lower the barrier voltage so that more charge carriers can diffuse into the base region and be swept into the collector by the voltage difference between the base and collector



Your statements and question are not very coherent. I don't know if you are describing a capacitdor or a PN junction. Again, charge carriers are not pushed or pulled from the emitter into the base by Vbe. They are moved by diffusion.



What can I say? Diffusion results from the concentration difference of holes and electrons of a PN junction. The P side has an abundance of holes, the N side has an abundance of electrons. They get together on the PN boundary and annihilate each other to form a depletion region.

Ratch

I think I get it now Ratch. It's Poisson’s Equation isn't it. That's what the quasi neutral regions are for and the total charge is zero which is why there is no electric field. Electric field only exists in the junction region because there are no free carriers in the neutral regions.
Thanks
Adam
Adam

 

[Arouse1973]

No, I have not. I like to keep myself in the real world. The following link shows what is typical from hundreds of sources of false information in textbooks and lectures.

Ratch

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html

I agree it's some what misleading. Their are loads of mistakes and misunderstandings in engineering here are a few I have found. What's also confusing is the picture of an NPN transistor shows the correct flow of linking current from the emitter to the collector. But we all use conventional current direction. So when you are new to electronics you could quite easily think of connecting the transistor the wrong way in the circuit.

THE ELECTRICITY INSIDE OF WIRES MOVES AT THE SPEED OF LIGHT

THE ELECTRIC ENERGY IN A CIRCUIT FLOWS IN A CIRCLE

THE TWO KINDS OF ELECTRICITY ARE "STATIC" AND "CURRENT

THE STUFF THAT FLOWS THROUGH WIRES IS CALLED 'ELECTRIC CURRENT

ELECTRIC CURRENT IS A FLOW OF ENERGY

ELECTRIC ENERGY IS CARRIED BY INDIVIDUAL ELECTRONS

BATTERIES STORE CHARGE, AND THIS CHARGE FLOWS IN WIRES

STATIC ELECTRICITY" IS A BUILDUP OF ELECTRONS

STATIC ELECTRICITY" IS ELECTRICITY WHICH IS STATIC

ELECTRIC POWER FLOWS FROM GENERATOR TO CONSUMER

LIGHT AND RADIO WAVES TRAVEL AT 186,000 MILES PER SECOND

ELECTRIC CHARGES ONLY FLOW ON THE SURFACES OF WIRES

ELECTRIC CHARGES ARE INVISIBLE

ATOMS HAVE EQUAL NUMBERS OF ELECTRONS AND PROTONS

HUMID AIR IS CONDUCTIVE

ELECTRIC ENERGY TRAVELS INSIDE OF WIRES

Thanks
Adam

 

[Ratch]

I think I get it now Ratch. It's Poisson’s Equation isn't it. That's what the quasi neutral regions are for and the total charge is zero which is why there is no electric field. Electric field only exists in the junction region because there are no free carriers in the neutral regions.
Thanks
Adam
Adam

I described what happens when the BJT is in the active region. I think you are referring to when the BJT is in equilibrium. This is a disparate subject as far as this thread goes. I suggest you refer to and gain understanding from a good book on semiconductor physics.

Ratch

 

[Ratch]

I agree it's some what misleading. Their are loads of mistakes and misunderstandings in engineering here are a few I have found. What's also confusing is the picture of an NPN transistor shows the correct flow of linking current from the emitter to the collector. But we all use conventional current direction. So when you are new to electronics you could quite easily think of connecting the transistor the wrong way in the circuit.


Thanks
Adam

I see you have been reading Bill Beaty's website. http://amasci.com/ The first thing you should do is not say "current flow". Current is charge flow, so current flow means "charge flow flow", which is redundant and ridiculous. You should instead say "charge flow" or "current exists".

Next you should stop saying that a capacitor is "charged". A capacitor is never charged. That is, it never contains more electrons or fewer electrons. A capacitor with a 1000 volts across its plates contains the same net charge that one with no voltage has. The difference is that the charges are distributed unevenly, with one plate containing more charges and the other fewer charges. It takes energy to redistribute those charges. That energy is stored in the cap's electrostatic field. A cap can be "charged with energy", not charge carriers. So you might as well say the cap is "energized" instead of the ambiguous word "charged".

How many times have I heard folks say that a cap allows alternating current to pass through it? Well, it doesn't. The dielectric of a cap is above all an insulator, and would be leaky and considered defective if it did. What is really happening is that the charges on the cap plates are redistributing themselves back and forth, thereby causing current to exist in the cap's branch, but not allowing any charge to flow through the cap itself.

Ratch

 

[Arouse1973]

Yes Ratch. It is very good I came across a pdf a few years ago Didn't realise where it came from at first. I have also contacted Ian Sefton a few times he is well cool.
Adam

 

[Arouse1973]

Anyway I get the message. So thanks for the help.

 

[(*steve*)]

I agree it's some what misleading. Their are loads of mistakes and misunderstandings in engineering here are a few I have found.

Here are a few others:

The voltage at all points along a perfect conductor is the same.

The current in a perfect conductor is the same at all points.

Actually, pretty much everything we know and understand about the physical world is an approximation or simplification. In some cases we *know* it, and we know the simplification works within certain constraints (like current control of a transistor). Others may simply be the best we understand the physics at present, but which are undoubtedly not 100% correct.

We could use Maxwell's equations to explain to a beginner how a simple circuit involving a battery, a switch and a lamp works. It would be correct. It would be really long. And it would be completely useless to the beginner in any practical sense. We could probably use some descriptions similar to those you have quoted as a means of simplifying the concepts.

Next you should stop saying that a capacitor is "charged". A capacitor is never charged.

Take Ratch's advice. There are other things you should never say are charged:

1) A glass ("The Master of Ceremonies asked us to charge our glasses, but mine was already charged!")
2) An offender ("Several rioters were charged with common assault")
3) A responsible person ("He was charged with the task of supervising the apprentice.")
4) A customer ("The customer was charged $5.")
5) A phone! ("The cell phone must be charged overnight before the first use.")
6) Military assets ("The cavalry charged into battle.")

No, the only allowable use of the word "charged" is in relation to something which can gain a net charge.

a) A comb ("I charged my comb on my hair this morning.")
b) Part of an ionic substance ("I charged all the sodium in my salt by dissolving it in water.")

That is, it never contains more electrons or fewer electrons.

Because, by this explanation an electron is not charged either, nor is any other particle (sorry physicists, you can't refer to charge on a particle any more).

Of course we can say a capacitor is charged. It is charged when the electric field across the plates caused by the imbalance of electric charge on each of those plates is sufficient to prevent that imbalance becoming greater at the applied voltage.

In this case "charged" has a meaning that is different to the one you describe, but one that is just as valid as 1 through 6 above.

How many times have I heard folks say that a cap allows alternating current to pass through it? Well, it doesn't.

Of course not. It also allows DC to pass through it for a short time

I have a black box. It has 2 wires coming out of it. You measure the current in each wire. You notice that (aside from issues relating to wavelength) the current in one wire is always the same magnitude and direction as in the other. In addition it is not always zero.

Does current pass through the device?

Or would you have it that I need to tag a particular electron with a red hankerchief and not say that current passes through the device until you see the red hankerchief appear at the other end?

If so, you also need to claim that with AC no current can pass through any conductor.

What if I were to tell you that this is a quantum device. It takes spontaneously created electron/positron pairs and uses the positron to annihilate an electron from one conductor whilst allowing the newly minted electron to appear in the other conductor. No electron which enters one side of the device *ever* leaves via the other conductor.

No electron ever crosses the gap. Is there current? Is there a flow of charge? Can you possibly have one without the other?

The dielectric of a cap is above all an insulator, and would be leaky and considered defective if it did. What is really happening is that the charges on the cap plates are redistributing themselves back and forth, thereby causing current to exist in the cap's branch, but not allowing any charge to flow through the cap itself.

And therefore the impedance of a capacitor is always infinite since without a flow of charge, we can have no current.

Does the current you measure in the leads of a tunnel diode all pass through the diode?

I can go on like this all day if you wish to cherry-pick the simplifications that satisfy your argument.

And hence my argument that you pick some reasonable simplification of the physical facts that fit most of the observed phenomena and which allow you to make reasonable predictions at the scale in which you operate. When explaining things to a beginner you intentionally take these and simplify them further. Then, as required, you add more and more detail. You don't start from the most detailed possible explanation first.

And that is why you'll find current control of transistors mentioned. It's a reasonable, understandable, workable, explanation that gives you a model which applies to almost all circuits.

Oh, and in post #98, after misquoting me, you admonished me for not reading something you posted in post #69. I had in fact read this, and following your further recommendation in post #91, I commented on it in post #92 (Although, given that posts 91 and 92 are so far separated, I can almost forgive you for not seeing it).

 

[Ratch]

Here are a few others:

The voltage at all points along a perfect conductor is the same.

The current in a perfect conductor is the same at all points.

Actually, pretty much everything we know and understand about the physical world is an approximation or simplification. In some cases we *know* it, and we know the simplification works within certain constraints (like current control of a transistor). Others may simply be the best we understand the physics at present, but which are undoubtedly not 100% correct.

Yes, and the point is?

We could use Maxwell's equations to explain to a beginner how a simple circuit involving a battery, a switch and a lamp works. It would be correct. It would be really long. And it would be completely useless to the beginner in any practical sense. We could probably use some descriptions similar to those you have quoted as a means of simplifying the concepts.

We could also use quantum mechanics to explain a simple circuit and make it even longer and more complicated. And the point is?

Take Ratch's advice. There are other things you should never say are charged:

1) A glass ("The Master of Ceremonies asked us to charge our glasses, but mine was already charged!")
2) An offender ("Several rioters were charged with common assault")
3) A responsible person ("He was charged with the task of supervising the apprentice.")
4) A customer ("The customer was charged $5.")
5) A phone! ("The cell phone must be charged overnight before the first use.")
6) Military assets ("The cavalry charged into battle.")

I never said that other nonelectrical meanings of charged should not be used. Make sure you specify that charging the cell means charging it with energy (energize).

No, the only allowable use of the word "charged" is in relation to something which can gain a net charge.

a) A comb ("I charged my comb on my hair this morning.")
b) Part of an ionic substance ("I charged all the sodium in my salt by dissolving it in water.")

Neither of the above have anything to do with capacitors.

Because, by this explanation an electron is not charged either, nor is any other particle (sorry physicists, you can't refer to charge on a particle any more).

I definitely did say electrons and holes are charged. But their charge is innate, not added or removed.

Of course we can say a capacitor is charged. It is charged when the electric field across the plates caused by the imbalance of electric charge on each of those plates is sufficient to prevent that imbalance becoming greater at the applied voltage.

Now you are confusing equilibrium with charge.

In this case "charged" has a meaning that is different to the one you describe, but one that is just as valid as 1 through 6 above.

Charge in the above case has no meaning with any definition.

Of course not. It also allows DC to pass through it for a short time

No current ever passes through a good capacitor.

I have a black box. It has 2 wires coming out of it. You measure the current in each wire. You notice that (aside from issues relating to wavelength) the current in one wire is always the same magnitude and direction as in the other. In addition it is not always zero.

Does current pass through the device?

I have no way of knowing without characterizing the current with respect to time and phase with voltage the way a network analyzer would.

Or would you have it that I need to tag a particular electron with a red hankerchief and not say that current passes through the device until you see the red hankerchief appear at the other end?

If so, you also need to claim that with AC no current can pass through any conductor.

Current can pass through a coil and a resistor, but not a capacitor.

What if I were to tell you that this is a quantum device. It takes spontaneously created electron/positron pairs and uses the positron to annihilate an electron from one conductor whilst allowing the newly minted electron to appear in the other conductor. No electron which enters one side of the device *ever* leaves via the other conductor.

No electron ever crosses the gap. Is there current? Is there a flow of charge? Can you possibly have one without the other?

Since a positron is an antimatter particle, and neither of us are particle physicists, I don't think you can realistically propose a situation, not can I answer it.

And therefore the impedance of a capacitor is always infinite since without a flow of charge, we can have no current.

There is a flow of charge, it just does not go through the capacitor.

Does the current you measure in the leads of a tunnel diode all pass through the diode?

It certainly does in one direction.

I can go on like this all day if you wish to cherry-pick the simplifications that satisfy your argument.

I am not simplifiying anything. I am correcting wrong statements and beliefs.

And hence my argument that you pick some reasonable simplification of the physical facts that fit most of the observed phenomena and which allow you to make reasonable predictions at the scale in which you operate. When explaining things to a beginner you intentionally take these and simplify them further. Then, as required, you add more and more detail. You don't start from the most detailed possible explanation first.

Who is doing that? Simplification is fine, as long as it does not lead to incorrectness.

And that is why you'll find current control of transistors mentioned. It's a reasonable, understandable, workable, explanation that gives you a model which applies to almost all circuits.

With respect to control, it is false, misleading, does ot explain diffusion, and only applies to transistor models. Remember, models only tell you what a device does within certain circumstances. Models do not tell you how a device works.

Oh, and in post #98, after misquoting me, you admonished me for not reading something you posted in post #69. I had in fact read this, and following your further recommendation in post #91, I commented on it in post #92 (Although, given that posts 91 and 92 are so far separated, I can almost forgive you for not seeing it).[/QUOTE]

I don't feel too bad since you have misconstured what I said about a capacitor being charged.

Ratch

 

[(*steve*)]

Now you are confusing equilibrium with charge.

No, I am explicitly not.

The convention is to say a capacitor is "charging" (or discharging -- depending on whether the voltage is trending away or toward 0) when it is not in a state of equilibrium, and "charged" when it is.

Charge in the above case has no meaning with any definition.

I'll tell that to the next Leyden jar I meet.

No current ever passes through a good capacitor.

I see you belong to the red handkerchief brigade.

I will agree that the same electrons that enter one side of a capacitor are unlikely to emerge from the other, but at the frequencies that we typically operate at, the current in both leads of a capacitor is the same. If a current is flowing, do I say it is flowing around the device?

I have no way of knowing without characterizing the current with respect to time and phase with voltage the way a network analyzer would.

That's a great answer if you ignore "You notice that (aside from issues relating to wavelength) the current in one wire is always the same magnitude and direction as in the other. In addition it is not always zero."

Since a positron is an antimatter particle, and neither of us are particle physicists, I don't think you can realistically propose a situation, not can I answer it.

It certainly does in one direction.

But you would also assert that without being a quantum mechanic )) you couldn't say whether the tunneling current does. So maybe only some of the current goes through the diode?

I would say if flows in both directions actually. Or do you assert that current never flows in the reverse direction through a diode?

 

[Ratch]

No, I am explicitly not.

The convention is to say a capacitor is "charging" (or discharging -- depending on whether the voltage is trending away or toward 0) when it is not in a state of equilibrium, and "charged" when it is.

I am saying that using the word "charging" in this case is a misnomer. The correct word is "energizing".

I'll tell that to the next Leyden jar I meet.

It is just another capacitor that can be energized.

I see you belong to the red handkerchief brigade.

I will agree that the same electrons that enter one side of a capacitor are unlikely to emerge from the other, but at the frequencies that we typically operate at, the current in both leads of a capacitor is the same. If a current is flowing, do I say it is flowing around the device?

Yes, there is current existing in both leads of the cap, but it is charge movement that goes/returns to/from the plates, but never passes through the dielectric. A cap is an energy storage device, not a resistor.

That's a great answer if you ignore "You notice that (aside from issues relating to wavelength) the current in one wire is always the same magnitude and direction as in the other. In addition it is not always zero."

Whatever. What is the point of that senario?

But you would also assert that without being a quantum mechanic )) you couldn't say whether the tunneling current does. So maybe only some of the current goes through the diode?

Where would the rest of the current go?

I would say if flows in both directions actually. Or do you assert that current never flows in the reverse direction through a diode?

Of course not. There is a saturation current in a junction diode that is thermally generated. It is prevalent in the reverse direction because no diffusion current exists during that condition, but is swamped out by the diffusion current in the forward direction.

 

[(*steve*)]

Here's a thread for you.

Can you assist here. I can't.

https://www.electronicspoint.com/threads/solar-charge-controller.268312/

 

[(*steve*)]

Whatever. What is the point of that senario?

You have a device in which charge flows in one end, and out the other, yet no electrons flow through it. Is it being "energised"? Or is current passing through it?

Where would the rest of the current go?

Well, I would say it goes through it, but there is a barrier through which the electrons can not pass. They tunnel -- which is totally distinct from passing through the barrier.

So here we have a gap through which no current passes, yet the device has (as you say) current passing through it.

Of course not. There is a saturation current in a junction diode that is thermally generated. It is prevalent in the reverse direction because no diffusion current exists during that condition, but is swamped out by the diffusion current in the forward direction.

But what of the capacitance of this device? Not all of the current actually passes through the diode according to you. Some of it "energises" it.

How do I distinguish between that current due to "energising" and that due to flow?

 

[davenn]

No current ever passes through a good capacitor.
Click to expand...

I see you belong to the red handkerchief brigade.

I will agree that the same electrons that enter one side of a capacitor are unlikely to emerge from the other, but at the frequencies that we typically operate at, the current in both leads of a capacitor is the same. If a current is flowing, do I say it is flowing around the device?

I have kept out of this discussion so far and watched the battle from the sidelines, somewhat bemused

yes, you must say its flowing around the device aka capacitor ... its flowing in and out of each plate but not across/between each plate

Dave

 

[Ratch]

You have a device in which charge flows in one end, and out the other, yet no electrons flow through it. Is it being "energised"? Or is current passing through it?

Well, I would say it goes through it, but there is a barrier through which the electrons can not pass. They tunnel -- which is totally distinct from passing through the barrier.

So here we have a gap through which no current passes, yet the device has (as you say) current passing through it.

Tunneling can occur in junction diodes, and is another name for the Zener process. In classical physics, a particle must have an energy greater than the barrier to appear on the other side. However, if the barrier is very thin, the carrier may tunnel through the barrier if a large nunber of electrons are available to tunnel on one side of the barrier, and a large number of empty states exist on the other side of the barrier to which the electrons can be received.

But what of the capacitance of this device? Not all of the current actually passes through the diode according to you. Some of it "energises" it.

How do I distinguish between that current due to "energising" and that due to flow?

Yes, what about the capacitance? It is never very large in semiconductors, and usually only important at high frequency. When tunneling starts, most all the charge does pass through the diode via an alternate path. There is a big conduction path through the depletion layer, so the junction capacitance is zilch. I hope you are not trying to compare the tight dielectric of a cap to the tunneling effect of a semiconductor.

Ratch

 

[Ratch]

I have kept out of this discussion so far and watched the battle from the sidelines, somewhat bemused

yes, you must say its flowing around the device aka capacitor ... its flowing in and out of each plate but not across/between each plate

Dave

To be more precise, charge (not current) is flowing and accumulating/depleting to/from the plates. What do you mean by flowing around the device? Is there a conduction path like a wire across the terminals of the cap?

Ratch