### Author Topic: How transistors work  (Read 23998 times)

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#### Legion

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##### How transistors work
« on: May 05, 2012, 03:34:40 pm »
I've been trying to figure out how transistors work, but unfortunately, all the resources I've found are really just a collection of facts, but not an explanation.  Knowing that a base current can modulate the emitter to collector current is great, but it doesn't really explain why it's able to do that.

So, any good resources online that explain what's going on inside a transistor at the electron level?  Also, does a transistor actually amplify current, or does it just allow us to control a larger current using a smaller one?

#### Kremmen

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##### Re: How transistors work
« Reply #1 on: May 05, 2012, 04:39:00 pm »
You could try starting from this: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html#c1
The browsing links are not too organized but you will find the topic tree containing the transistor as well.
The explanations are necessarily condensed as the full dissemination takes several volumes. But maybe you get the idea at least.

Wouldn't it be nice if a transistor actually amplified the current? Instant free energy... No, it just modulates a larger current with a smaller one.
Nothing sings like a kilovolt.
Dr W. Bishop

#### vxp036000

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##### Re: How transistors work
« Reply #2 on: May 05, 2012, 05:04:09 pm »
I think the reason you haven't found much online is that a strong background in quantum mechanics (read: math!) is needed to describe what's happening inside a transistor.  There are two basic transistors: FETs and BJTs, FETs will need to wait for another time.

We'll start with the BJT, the NPN transistor.  In series, there is an N doped semiconductor, P doped semiconductor, and another N doped semiconductor.  The first N doped material goes to Vcc, the P doped material goes to the base contact, and the second N doped material is connected to ground.  When I increase the base voltage, the depletion region around the PN junction from base to ground decreases, forward biasing the junction and increasing current flow.  The reverse biased NP junction from Vcc to base makes for an excellent electron sink.  This is because reverse biasing the junction increases the width of the depletion region, so any minority carrier electons in the P material experience strong acceleration toward Vcc.  So we see that increasing the base voltage increases the current flow; so we have a voltage controlled current source.

If I lost you already, I suggest first studying physics behind the basic PN diode.  BJTs are simply an extension of the concept.

I've been trying to figure out how transistors work, but unfortunately, all the resources I've found are really just a collection of facts, but not an explanation.  Knowing that a base current can modulate the emitter to collector current is great, but it doesn't really explain why it's able to do that.

So, any good resources online that explain what's going on inside a transistor at the electron level?  Also, does a transistor actually amplify current, or does it just allow us to control a larger current using a smaller one?

#### TerminalJack505

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##### Re: How transistors work
« Reply #3 on: May 05, 2012, 06:28:30 pm »
I should warn you now if you are just getting into electronics.  If you think you will be able to get a clear picture about what's going on at a physical level you are likely to be disappointed.

Much of the work in electronics was done before the electron was discovered and before QED was developed.  The work done through the centuries has a mix of theories and physical models.  The earlier theories don't even attempt to actually describe what happens at a physical level.  This can be frustrating if you are  "visually oriented" or just have to know what is truly happening at a physical level.

Just as one example, you will find that the physical forces involved in electronics can be described in various references by any of the following:

• Newton's mysterious "force at a distance" (mechanical analog, E.M.F., for example.)
• Classical field theory (properties of empty space, waves, etc., flux density, for example.)
• Field particle theory (QED, our current understanding.)

So, when you learn about transformers you are likely to learn about 'flux.'  There is no such physical thing as flux.  It just happened to be a good model for what scientists where trying to describe.  (They didn't know about electrons and their properties, let alone the force carrier, the virtual photon, at the time.)

#### vxp036000

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##### Re: How transistors work
« Reply #4 on: May 05, 2012, 06:37:29 pm »
I think an understanding at the physical level isn't much of a stretch with quantum mechanical models.  Anything less is nothing but an analogy and it's not hard to find common scenarios in which the analogy is completely invalid.

I should warn you now if you are just getting into electronics.  If you think you will be able to get a clear picture about what's going on at a physical level you are likely to be disappointed.

Much of the work in electronics was done before the electron was discovered and before QED was developed.  The work done through the centuries has a mix of theories and physical models.  The earlier theories don't even attempt to actually describe what happens at a physical level.  This can be frustrating if you are  "visually oriented" or just have to know what is truly happening at a physical level.

Just as one example, you will find that the physical forces involved in electronics can be described in various references by any of the following:

• Newton's mysterious "force at a distance" (mechanical analog, E.M.F., for example.)
• Classical field theory (properties of empty space, waves, etc., flux density, for example.)
• Field particle theory (QED, our current understanding.)

So, when you learn about transformers you are likely to learn about 'flux.'  There is no such physical thing as flux.  It just happened to be a good model for what scientists where trying to describe.  (They didn't know about electrons and their properties, let alone the force carrier, the virtual photon, at the time.)

#### free_electron

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##### Re: How transistors work
« Reply #5 on: May 05, 2012, 07:50:28 pm »
sigh.. so all we have so far is the belitteling of people that have no math or physics understanding (the usual 'talking down'on people, and 'look at us we have a degree')
But none have given an explanation on how it works...

-edit- while i was typing this up even more 'fuzz' was added...

here we go : (without the math, QED bullshit overload )

There are two types of material :

N material is a smiconductor that has an excess of free electons
P material is a semiconductor that has a shortage of electrons ( sometimes explained as 'an excess of holes'. in reality there are no 'holes' , ony lack of electrons )

N and P material are constructed by 'seeding' ( the technical process is called implantation ) a donor material into the silicon ( or germanium or other semiconductor ). Silicon has 4 electrons on its outer shell but would really want to have 8.. so it readily forms bonds with other silicon atoms. We can create a 'free electron' by injecting a donor material that has 5 electrons on its outer shell ( like Boron. the silicon will combine with the Boron and 'share electrons'. this fifth electron is now 'free'. It is still bound by atomic forces to the phosphorous but it is also highly mobile : it can be 'knocked off easily' that is why we call it free.
We can also create bonds with an element that has only 3 electrons on its outer shell , like phosphorous. Here the semiconductor will bond and have a link with 7 electrons ( 4 of its own and 3 donors from the implanted material. So it is one short from an ideal state, but it is happier than ith only 4.

The material that is one electron short is P material ( there is a missing electron or a 'hole' there to fit one in )
The material that has an excess of electrons is N material.

Right , now that we know this we can take a look at the transistor structure.
The classic bipolar transistor is a 3 layer structure of N and P material. The denotation NPN and PNP tell you how the layers are ordered.
Lets take a look at an NPN stack.

Before we delve in in :
the properties of a conductor material is that electrons can flow freely. if all electrons are trapped you have an isolator.

So we stack a bit of n material on top of P material on top of another bit of N material. And then magic happens.
At the intersection of these mateirals the free electrons fomr the N material combine with the free 'holes' of the P material and you get actually a thin region where all free electrons have fallen in a hole. ( this is called the depletion layer ) and no further electrons can cross this barrier.

In schematic form
Code: [Select]
 NNNNDPPDNNNN NNNNDPPDNNNN NNNNDPPDNNNN
N = n material full of free electrons
D = depletion zone : n material free electrons have fallen in p material holes
P = p material 'short of electrons

So what happened here is that , free electrons form N regions have fallen in 'holes' in the P region . This has trapped them and created two depletion layers. Since the electrons are trapped you have effectively created an isolator there. The reason this forms only a thin layer is simple : as electrons combine with holes the 'pressure' decreases. Let me see how i can explain this one:

We know that like charges repel each other and dislike charges attract each other. So , in an area where you have an excess of electrons , they are all trying to push each other away. if you give them a path to escape they will go that direction. So here is this one side that touches the P material full of nice holes they can fall into : and away they go. They happely move that way , fall in a hole and get trapped. The electrons behind them can still skip over because the ones behind those are still pushing. But eventually the depletion zone is wide enough , and enough electrons have been removed from the n region that the reminaing electrons have not enough 'push' to keep this process going and it stops. It stops , provided we do not apply an external source of pressure.

And this is what we will do : I am going to apply an 'electron source' to the right hand terminal to give a bit more 'pressure'  . An electron source is the negative terminal of a power supply as electrons carry negative charge.
To close the loop i will apply the positive terminal of the same supply to the P material.

Code: [Select]
    NNNNDPPDNNNN                  ==NNNNDPPDNNNN================     NNNNDPPDNNNN                ||       ||                       ||       ||                       ||       ||                       ||        =======(+)source(-)======so , by feeding more electrons into the right hand terminal i create more 'pressure' and  current will begin flowing ( the arrow above shows the way they flow). I am pulling the electrons out the P material region.

For clarity sake , lets slap a name on these terminals shall we ? Since the element i connected my 'electron source' to is 'emitting' them, i shall call this terminal the emitter. pretty logical.
The terminal i am pulling the electrons from i shall call... the base. No, not the collector.... got you there ! I will explain later why this is the 'base'.

So , by sending electrons in the emitter , and pulling them out of the base i create an electron flow. This is the base-emitter current ( in electron flow the emitter-base current, but a it was not known electrons were negative in the olden days we assumed current flows positive to negative... )

Right. Now i will attach another power source. A source that will feed even more electons in to the emitter but , this time, will attempt to pull them out of the third terminal. The 'collector' terminal , as this is where we will tempt to collect them.

Code: [Select]
  ================(+)source(-)=====||                                ||||  NNNNDPPDNNNN                  ||  ==NNNNDPPDNNNN==================O     NNNNDPPDNNNN                  ||         ||                       ||         ||                       ||         ||                       ||          =======(+)source(-)======
If i am not pulling anything out of the base i have those two darned depletion layers in the way that prevent current from flowing. if i start pulling some electrons out of the base the depletion area is being broken down (remember the depletion area is the area where no movement is possible because there is 'pressure' balance... ) and the electrons start flowing.
if i start pulling harder and harder i can move the depletion areas so much that they almost touch each other. In the mean time , my other 'power source ( the one between collector and emittor) has been wicking away electrons from the collector region ( in rest there is a surplus of electrons there. Because we 'doped' this material duering construction remember ? )

so we come to a point that electrons are being wicked from emitter into base , and they get so close to an area where there is even a bigger 'void' of electrons ( the collector is more positive than the base. More positive means less electrons avaialble , or more holes ) so electrons that were destined to go into the base actually start flowing into the collector region where they are whisked away by the power suource there. And there you have the transistor function.

The harder you pull on that base , the more electrons you pull out of the emittor. if they come close enough to the collector area they are attracted there becasue there is an even bigger 'void' of electrons there. so , by 'steering' the flow in the base you steer the flow in the collector. Amplification !

So the base current actually moves these 'zones' until they touch and then celectrons start flowing from emitter to collector. if i stop pulling there the depletion areas move inside the structure and the flow stops.

Now, there is more than meets the eye. This trickery only works under certain contitions.
That right hand terminal ( the emitter ) is actually heavily seeded with electrons.
The base is only mildly made void of electrons the collector is also only mildly seeded.

So there is more 'pressure' in the emitter than in the collector.
This base layer is also very very thin compared to the other two.

This helps the electron run into the collector as there is only so much that can flow into the base ( there are not enough holes there for all of them to fit as it is only mildly seeded with holes )
And since the collector area is only mildly seeded this area is very fast depleted by the higher positive voltage applied there. So the electron coming out of the emitter can go 2 ways : into the base or into the collector. ( remember the collector is beeing sucked 'dry' by the applied power supply . a suply that is at a higher level than the level at the base) and they take the path of least 'resistance' : the path where there are the most missing. in essence that collector 'N' region actually becomes a 'p' region while the transistor has current flowing through it.

This is your very very basic operating of a bipolar transistor. To make a PNP redo this explanation but swap 'electrons' with 'holes' and swap 'positive' with negative.

Now , on the subject of this 'base' terminal. Here's where that comes from.
The first transistor was not made out of N and P material. You can actually make a depletion area between a doped semiconductor and a metal. you don't need tow doped semiconductors. Actually the word semiconductor is used to denote an element where you can change the conductivity of. it can be isolating , or it can be conducting , dpeending on what you do with it. a metal is always conducting. an isolator alwasy iolating ( unless you 'force' charge across with extremely high voltages (voltage = pressure ))

But metal can be an electron donor.. tis is actually the principle behind a shottky diode. a classic diode is an N and P region where as a schottky diode uses only an N region and a strip of metal.
the first transisotr was a strip of P material that was laid on top of a copper plate. this strip of p material formed thus the 'base' of the transistor construction. And there you have it : the 'base'
the emittor and collector were made by putting a strip of gold foil on a triangular piece of glass. this piece of glass was pressed down with a screw onto the base. the pressure of the sharp edge did two things : it cut the goild foil in hlf and pressed the extremities into the germanium 'base'  (they used gemranium at the time )
the strip of gold 'feeding' electrons was the 'emitter' , the strip collecting them the 'collector'

Code: [Select]
collector--     ---- emitter           \   / base    ___\ /____               and this is also where the transistor symbol comes from... the line is the base and you have two electrodes under a 45 degree angle ( they used a 90 degree corner of glass , put that under 45 degrees and pressed down , snipping the gold foil in half and creating the two depletion regions of gold-germanium )
I scavenged two links from the internet that show this first transistor ( replica's )

This one below is high resolution. You see at the bottom a slab of copper , on top of that the slab of germanium and then the triangular piece of glass with the gold foil on edge ( they metallized the glass so they could solder it to that 'wavy bit' whch is actually a spring pressing the glass down on the germanium slab.
http://www.porticus.org/bell/images/transistor1.jpg

We use the arrow to indicate where the emitter sits.
Now , that first transisotr worked both ways. there wa sno difference between emitter and collector apart form where they applied the correct polarity. This came only later as tey sought to improve performance. that is where they started making the assymetrically shaped construction of having a heavily doped emittor and a weakly doped collector. but that is another story ...

So there you have it : the transistor in simple terms , without requiring a degree in maths and physics. You can of course now slap on the maths and start calculating the field strengths and electron levels and all the other stuff , and figure out what is the optimum layer thickness, implantation strength but that is just a numerical representation of what is happening.
Electrons are very simple elements that never went to school and don't know anything about maths or physics. They repel each other and if they see an area where there is 'room' they simply go that way.

No doubt there are going to be people that will start complaining about quantum effects and other things. The above explanation is according to the 'electron model'. A Model that is understandable without 'fuzz'. If you think you can do better , please feel free to write a posting here with a more accurate model.

« Last Edit: May 05, 2012, 08:24:54 pm by free_electron »
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).

#### IanB

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##### Re: How transistors work
« Reply #6 on: May 05, 2012, 08:38:30 pm »
Bravo, free_electron, an explanation without mathematics!

This is important, because if you think you need to resort to mathematical models to explain how something works, you don't really understand it. Mathematical models are used to calculate the details of what something does, not to explain how it does it.

By the way in case it wasn't obvious, N type material is so called because it has negative charge carriers, and P type material because it has positive charge carriers. Then you can view the depletion layer as being a zone where the negative and positive charge carriers cancel each other out and leave no charge carriers. No charge carriers in a material makes it an insulator.

#### vxp036000

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##### Re: How transistors work
« Reply #7 on: May 05, 2012, 08:55:03 pm »
The difficulty a lot of people run into when they try to avoid the math is that they come up with an explanation that violates some fundamental law of physics, without realizing it.  free_electron's explanation is a great analogy for many situations, but it's important to realize the limitations of the analogy.  Furthermore, I would say that someone doesn't truly understand something unless they can explain it in terms of fundamental laws of physics, which, by the way, are defined as mathematical models.

For example, the analogy doesn't explain band gap references, why transistors perform so poorly when collector and emitter are interchanged, thermal runaway, the active region of a BJT (needed for any amplifier to work), etc.  The mathematical model explains all of these exceptions without needing to resort to another analogy and having to explain why the analogy is no longer valid.

Bravo, free_electron, an explanation without mathematics!

This is important, because if you think you need to resort to mathematical models to explain how something works, you don't really understand it. Mathematical models are used to calculate the details of what something does, not to explain how it does it.

By the way in case it wasn't obvious, N type material is so called because it has negative charge carriers, and P type material because it has positive charge carriers. Then you can view the depletion layer as being a zone where the negative and positive charge carriers cancel each other out and leave no charge carriers. No charge carriers in a material makes it an insulator.
« Last Edit: May 05, 2012, 09:01:23 pm by vxp036000 »

#### Kremmen

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##### Re: How transistors work
« Reply #8 on: May 05, 2012, 09:10:10 pm »
free_electron: I actually read through your post. Not bad. It really should be enough for anyone who wants a basic grasp of a BJT. Somehow i felt the OP had reached this level but still wanted to know how it "really" works.
I didn't see so much belittling in the previous posts - certainly i didn't try to be that way. The fact just is that if you want to know what is "really" going on, then enter the next level of description. Whether that is useful for a practicing designer i am not at all sure, but that was the way the question was put.
While i mostly agree with IanB regarding narrative explanation of things, there is a limit after which it does not help further. Narration is qualitative, not quantitative. A transistor circuit designer will need things like hfe, VCEmax etc etc on top of the explanation of how it all works. He won't find quantum mechanical explanations, much less formuals very useful because they describe the same thing on a different level that is not relevant for practical design. Yet the level is there. The problem with everything quantum is that it wasn't designed to be readily understandable. Some of it is literally beyond reason as i am sure you know (just consider the photon diffraction experiment http://en.wikipedia.org/wiki/Double-slit_experiment). Still, the bottommost explanation is the only one that covers every aspect, simplified models will miss parts of the reality.
Nothing sings like a kilovolt.
Dr W. Bishop

#### IanB

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##### Re: How transistors work
« Reply #9 on: May 05, 2012, 09:43:44 pm »
The difficulty a lot of people run into when they try to avoid the math is that they come up with an explanation that violates some fundamental law of physics, without realizing it.  free_electron's explanation is a great analogy for many situations, but it's important to realize the limitations of the analogy.  Furthermore, I would say that someone doesn't truly understand something unless they can explain it in terms of fundamental laws of physics, which, by the way, are defined as mathematical models.

For example, the analogy doesn't explain band gap references, why transistors perform so poorly when collector and emitter are interchanged, thermal runaway, the active region of a BJT (needed for any amplifier to work), etc.  The mathematical model explains all of these exceptions without needing to resort to another analogy and having to explain why the analogy is no longer valid.

That's not really a sufficient view of the world. As Kremmen says, there are layers of explanation and you need to pick the layer that suits your purpose. There is no need to pick a more detailed model to satisfy a simpler need. Engineering is all about picking the right level of abstraction to suit the task at hand. Also, fundamental laws of physics are only "fundamental" until someone finds more fundamental laws underneath.

Someone above suggested that magnetic flux is not "physically real", but that virtual photons somehow are. Isn't there a certain amount of absurdity in that? Philosophers can spend forever debating what is "real" and what is not, but when it comes down to it all mathematical descriptions of physical behavior are abstractions. Mathematical models create an analogy of physical systems using a formal technical language, where we hope that what the model does in the abstract will be reflected in real life. We have got quite far down into the fine detail these days, but that does not mean we are finished.

free_electron: I actually read through your post. Not bad. It really should be enough for anyone who wants a basic grasp of a BJT. Somehow i felt the OP had reached this level but still wanted to know how it "really" works.
I didn't see so much belittling in the previous posts - certainly i didn't try to be that way. The fact just is that if you want to know what is "really" going on, then enter the next level of description. Whether that is useful for a practicing designer i am not at all sure, but that was the way the question was put.
While i mostly agree with IanB regarding narrative explanation of things, there is a limit after which it does not help further. Narration is qualitative, not quantitative. A transistor circuit designer will need things like hfe, VCEmax etc etc on top of the explanation of how it all works. He won't find quantum mechanical explanations, much less formuals very useful because they describe the same thing on a different level that is not relevant for practical design. Yet the level is there. The problem with everything quantum is that it wasn't designed to be readily understandable. Some of it is literally beyond reason as i am sure you know (just consider the photon diffraction experiment http://en.wikipedia.org/wiki/Double-slit_experiment). Still, the bottommost explanation is the only one that covers every aspect, simplified models will miss parts of the reality.

This is the key thing, about finding levels of description that meet your needs.

I agree about qualitative vs quantitative. That's what I meant when I said mathematical models serve the purpose of "what will happen" rather than "how it will happen". Understanding how things happen is a much deeper and more complex problem that requires insight, scientific intuition, abstraction, analysis and many other skills beyond computation with formulas.

#### T4P

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##### Re: How transistors work
« Reply #10 on: May 05, 2012, 09:52:36 pm »
http://amasci.com/amateur/transis.html

Don't let anyone tell you they are current controlled

#### free_electron

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##### Re: How transistors work
« Reply #11 on: May 05, 2012, 10:05:36 pm »
band gap references,
the explanation is on transistors , not bandgaps. if you want to study bandgaps you need to delve into chemistry(how the atom is built) and physics (how electrons interact with the nucleus of an atom)

Quote
why transistors perform so poorly when collector and emitter are interchanged
that is actually explained... has to do with the different doping strentgh between collector and emittor. in a point contact or cat whisker transistor there is no 'emitter and collector.. you find out which is which by TRYING it out... you can't control how hard you press and where you create the better doping. only when planar transistors were invented and they performed the doping during deposition in the oven ( only later came ion implantation machines. i did maintenance on those beasties for a while. i actually started in the waferfab maintaining and repaireing the ion implanters and plasma etchers... that was fun. very clever technology like the mass analyzer to select what you implant , the dosimetry technique used. rotating magnetic fields to create uniform plasma dispersion )

Quote
thermal runaway,
has nothing to do with the fundamental working.
you are adding 'fuzz' to the basic operation. if you want to add fuzz : how about other impurities ? they create noise ? how about tunneling effects ? how about 'hot carrier injection ? . it does no good to
Quote
The mathematical model explains all of these exceptions without needing to resort to another analogy and having to explain why the analogy is no longer valid.
try to teach someone math by starting with integrating quadratic equation and see where you end up... nowhere.
Start with numbers and + - * / then work your way up. This is what i did here. Starting with the absolute basics.
Now you can explore if you want why electrons repel each other , how the recombination works on a physical level. Then you can add on what other dopants there are , when to use them andwhy. and then you can start looking at geometry of the electrodes. and you can pull in field theory and other stuff as well. But it does no good to start with a 500 page manual of equations.

Quote
Bravo, free_electron, an explanation without mathematics!

This is important, because if you think you need to resort to mathematical models to explain how something works, you don't really understand it. Mathematical models are used to calculate the details of what something does, not to explain how it does it.
that is how i approach mathematics. You need those to work out the details , not the 'big picture'.

@kremmen : the 'sigh' was not pointed at you.. it was pointed at the posts that started with "you need 'mathematics' and 'nuclear physics' and 'quantum mechanics' ... so we won't even bother to try to explain it, you wouldn't understand".

hFe , VceMax , IbMax et al are properties of a transistor that are determined purely by construction. hFe is balance between the depletion zones. Vcemax is how wide you can make the depletion zone ( wider isolation means you can hold higher voltage without flashtrough. IbMax is determined by thickness of the base layer and so on. This is stuff you can 'tack on' later.

Here is a nice one that i used to know the explanation for but i can't remember: if you reverse polarize the BE junction and you hit a roughly 6 volts threshold you will succeed in 'poisoning' the gate. This creates some kind of physical process whereby fre electrons become permanently trapped in the gate layer (i believe it is a kind of electromigration, but i can't remember exactly how it works) . the result is that the beta (hFE) permanently decreases. The longer you keep this up the lower the beta becomes. this goes on until the transistor is eventually destroyed. this process is irreversable. that is why in certain circuits you see actually two series diode ( or a zener) placed between base and emittor... this prevents this reverse voltage level from ever reaching the 'zener' voltage of the BE junction and causing this irreversible degradation.

my explanation is a base explanation. the finer details require much more work since you need to pull in physics , the atom model , chemistry ( because there are impurites during implant and deposition. no matter how good your mass selector is you are shooting hydrogen in that crystal. implantation also shatters the crystal structure and you need a healing process in an oven afterwards. the same process is used for 'drive-in' : get the implanted material to diffuse deeper. this causes a thicker layer. : thicker 'wire' = more electorn handling capability. More electrons per second = more amperes ..

Everything is interconnected in the universe like a ball of yarn , but you can only pick it apart by starting with the first thread. Saying 'it's a ball of yarn and very complex , you wouldn't understand' gets nobody anywhere. You need to actually sit down and begin somewhere.. too bad the fora are full of people that don't do this... this does not stop me from trying call me an idealist.

Besides, at the end .. you know how transistors are really designed ? By putting a bunch of geometries on mask, running the wafer in the selected technology process and then trying them out on a curve tracer. You run a bunch of corner lots ,burn them in a bit ,  plot some curves , look where the bell sits and you throw this in a library. presto. now you can go off and design something practical with these things. Slap in a spice model extracted from the plots and you can even simulate your design. Heck i have characterized once a full ibrary of transistors , even full opamps. We had all these variations. They were built , burned in and then sent through a test setup. I wrote the software that drove the current sources and swept them. i had a stack of keithley 2400 sourcemeters, some 34401 multimeters and a big multiplexer array. That hing spat out tons of curves. This was later crunched, overlap was removed and distinct 'transistors were selected. These geometries were then fully specced ( fT , destructive testing to see where they fried ) and later the spice model was built. This ended up in a design library and then we could make chips with that. it took months to build the design lib.
i made a testjig for the opamps where i could do cmrr psrr , bandwidth , unity gain bandwidth, gain. it took over  10 minutes to fully sweep one ( on different vcc's , temperature sweeps ) and we had thousands of those prototypes. i had 5 setups running in parallel with 3 techs ( the fab had 24/7 staff in in 3 shifts ). poor guys had to put a chip in the socket , close the metal box ( faraday cage) and press start. 8 hours non stop. week after week. We sent them a big box of bagels one day because we felt sorry for them

It is today STILL done that way. Whenever the process is scaled everything is re-characterized to build the design library. Companies that built 3 pin transisotrs also do this. We have a whole lab with 300 people that only does this. make variations, spin a wafer , select the best and then torture them.. I once saw  testing on one of those 'puck' thyristors destined for traction control (locomotives)... it was specced for 20 kiloampere... their task was to see when it would actually fry ... it violently exploded at about 35 kiloampere ... ( i was there to have a protection structure tested and analysed. it had failed and they needed to cut it up. The structure was supposed to withstand metallic discarge on phone lines but it failed one of the ISPRC tests. i was working on the analog frontend of ADSL modems at the time. was 1996 or so... long ago)

No designer builds transistors at the molecular level (unless you are a transistor designer.. ) you use what is in the 'pool'. Practical design is a collaboration and building on other subunits. There is no point in re-inventing the bloody wheel over and over. if i need a timer circuit i am not going to build a 555 by smelting and refining sand i found on the beach , and then try to construct a wafer etc ... i buy a 555 , read the datasheet and off you go. If you are tasked to design a timer that delays 10 seconds you don't need to be able to calculate the weak nuclar bond...
there is such a thing as 'practicality'.

anyway. just my two cents of trying to help answer questions that people have about electronics, in an understandable manner. the world is already full of questons on how to calculate anLED series resistor and hard-whino 'programmers'. I'm trying to 'lift' it a bit beyond that ,without spooking people too much... We live in a wonderous universe. Let's go out and explore !

@dave : that is a well know 'alternate' explanation.
If you read mine attentively you will see that i never use the word 'base current'. It is the voltage difference ( potential) that causes the electrons to flow , and the electrons are charge carriers. it i indeed the pressure ( the 'voltage' , technical definition 'potential' that cases charge to move ) i control the direction it is allowed to flow by applying pressure ( or void thereof ) to the base. i will get electrons out of the base ( or in the base-e) because there is a conductive pathway ( something that is NOT there in a mosfet. There is a connection in a J-Fet though, but we polarize it in reverse to create the barrier that way. ).

if you look at transistor plots , some do specify the vbe/Ibe plot.. just like you can make an Ib/Ic plot you could make a Vbe/Ic plot ... ( substitute the Ib scale with the Vbe/Ic plot.. and you got it.)
it is all interconnected. you can explain it in voltage domain or in current domain. for bipolars it makes more sense to explain it in current domain... for a mosfet it works better in voltage domain.
pure practicality...
« Last Edit: May 06, 2012, 01:58:05 pm by free_electron »
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#### westfw

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##### Re: How transistors work
« Reply #12 on: May 06, 2012, 05:26:45 am »
MOSFETs are even easier to explain at a qualitative level.
The Source to Drain path is a "channel" that can contain current carriers (as per FreeElectron's "doping" description of silicon.)  A voltage (charge) on the gate sets up an electric field (Thus "Field Effect Transistor") can move those current carriers into and out of the channel, just because "like charges repel and opposite charges attract."

I vaguely recall my BSEE jumping pretty quickly from quantum mechanical explanations of theoretical transistors to measured properties of real transistors, without doing all the math needed to explain the exact behavior of real devices (and you look at the range of values permitted by a device spec sheet, and you realize that there are a LOT of complications in there.  Like FE says, it's all "layers" of understanding.

#### Mechatrommer

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• reassessing directives...
##### Re: How transistors work
« Reply #13 on: May 06, 2012, 09:07:43 am »
+2 bravo to free_electron... i just read about the section in academia book, pretty much thats exactly how he explained it. except i think (havent finish read it) free_electron got more details in the explanation. i hereby award free_electron with "nobel excellency in ee lecturacy" . i believe most people lost their way by thinking math describes everything... i believe math is built based on observation of the real world, not the other way around.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock

#### M. András

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##### Re: How transistors work
« Reply #14 on: May 06, 2012, 11:21:01 am »
band gap references,
the explanation is on transistors , not bandgaps. if you want to study bandgaps you need to delve into chemistry(how the atom is built) and physics (how electrons interact with the nucleus of an atom)

Quote
why transistors perform so poorly when collector and emitter are interchanged
that is actually explained... has to do with the different doping strentgh between collector and emiitor. in a point contact or cat whicsker transistor there is no 'emitter and collector.. you find out which is which by TRYING it out... you can control how hard you press and where you create the better oping. only when planar transistors were invented and they performed the doping during deposition in the oven ( only later came ion implantation machines. i maintenacne on those beasties for a while. i actually started in the waferfab maintaining and repaireing the ion implanters and plasma etchers... that was fun. very clever technology like the mass analyzer to select what you implant , the dosimetry technique used. rotating magnetic fields to create uniform plasma dispersion )

Quote
thermal runaway,
has nothing to do with the fundamental working.
you are adding 'fuzz' to the basic operation. if you want to add fuzz : how about other impurities ? they create noise ? how about tunneling effects ? how about 'hot carrier injection ? . it does no good to
Quote
The mathematical model explains all of these exceptions without needing to resort to another analogy and having to explain why the analogy is no longer valid.
try to teach someone math by starting with integrating quadratic equation and see where you end up... nowhere.
Start with numbers and + - * / then work your way up. This is what i did here. Starting with the absolute basics.
Now you can explore if you want why electrons repel each other , how the recombination works on a physical level. Then you can add on what other dopants there are , when to use them andwhy. and then you can start looking at geometry of the electrodes. and you can pull in field theory and other stuff as well. But it does no good to start with a 500 page manual of equations.

Quote
Bravo, free_electron, an explanation without mathematics!

This is important, because if you think you need to resort to mathematical models to explain how something works, you don't really understand it. Mathematical models are used to calculate the details of what something does, not to explain how it does it.
that is how i approach mathematics. You need those to work out the details , not the 'big picture'.

@kremmen : the 'sigh' was not pointed at you.. it was pointed at the posts that started with "you need 'mathematics' and 'nuclear physics' and 'quantum mechanics' ... so we won't even bother to try to explain it, you wouldn't understand".

hFe , VceMax , IbMax et al are properties of a transistor that are determined purely by construction. hFe is balance between the depletion zones. Vcemax is how wide you can make the depletion zone ( wider isolation means you can hold higher voltage without flashtrough. IbMax is determined by thickness of the base layer and so on. This is stuff you can 'tack on' later.

Here is a nice one that i used to know the explanation for but i can't remember: if you reverse polarize the BE junction and you hit a rouglhy 6 volts threshold you will succeed in 'poisoning' the gate. This creates some kind of physical process whereby fre electrons become permanently trapped in the gate layer (i believe it is a kind of electromigration, but i can't remember exactly how it works) . the result is that the beta (hFE) permanently decreases. The longer you keep this up the lower the beta becomes. this goes on until the transistor is eventually destroyed. this process isirreversable. that is whay in certain circuits you see actually two series diode ( or a zener) placed between base and emittor... this prevents this reverse voltage level from ever reaching the 'zener' voltage of the BE junction and causing this irreversible degradation.

my explanation is a base explanation. the finer details require much more work since you need to pull in physics , the atom model , chemistry ( because there are impurites during implant and deposition. no matter how good your mass selector is you are shooting hydrogen in that crystal. implantation also shatters the crystal structure and you need a healing process in an oven afterwards. the same process is used for 'drive-in' : get the implanted material to diffuse deeper. this causes a thicker layer. : thicker 'wire' = more electorn handling capability. More electrons per second = more amperes ..

Everything is interconnected in the universe like a ball of yarn , but you can only pick it apart by starting with the first thread. Saying 'it's a ball of yarn and very complex , you wouldn't understand' gets nobody anywhere. You need to actually sit down and begin somewhere.. too bad the fora are full of people that don't do this... this does not stop me from trying call me an idealist.

Besides, at the end .. you know how transistors are really designed ? By putting a bunch of geometries on mask, running the wafer in the selected technology process and then trying them out on a curve tracer. You run a bunch of corner lots ,burn them in a bit ,  plot some curves , look where the bell sits and you throw this in a library. presto. now you can go off and design something practical with these things. Slap in a spice model extracted from the plots and you can even simulate your design. Heck i have characterized once a full ibrary of transistors , even full opamps. We had all these variations. They were built , burned in and then sent through a test setup. I wrote the software that drove the current sources and swept them. i had a stack of keithley 2400 sourcemeters, some 34401 multimeters and a big multiplexer array. That hing spat out tons of curves. This was later crunched, overlap was removed and distinct 'transistors were selected. These geometries were then fully specced ( fT , destructive testing to see where they fried ) and later the spice model was built. This ended up in a design library and then we could make chips with that. it took months to build the design lib.
i made a testjig for the opamps where i could do cmrr psrr , bandwidth , unity gain bandwidth, gain. it took over  10 minutes to fully sweep one ( on different vcc's , temperature sweeps ) and we had thousands of those prototypes. i had 5 setups running in parallel with 3 techs ( the fab had 24/7 staff in in 3 shifts ). poor guys had to put a chip in the socket , close the metal box ( faraday cage) and press start. 8 hours non stop. week after week. We sent them a big box of bagels one day because we felt sorry for them

It is today STILL done that way. Whenever the process is scaled everything is re-characterized to build the design library. Companies that built 3 pin transisotrs also do this. We have a whole lab with 300 people that only does this. make variations, spin a wafer , select the best and then torture them.. I once saw  testing on one of those 'puck' thyristors destined for traction control (locomotives)... it was specced for 20 kiloampere... their task was to see when it would actually fry ... it violently exploded at about 35 kiloampere ... ( i was there to have a protection structure tested and analysed. it had failed and they needed to cut it up. The structure was supposed to withstand metallic discarge on phone lines but it failed one of the ISPRC tests. i was working on the analog frontend of ADSL modems at the time. was 1996 or so... long ago)

No designer builds transistors at the molecular level (unless you are a transistor designer.. ) you use what is in the 'pool'. Practical design is a collaboration and building on other subunits. There is no point in re-inventing the bloody wheel over and over. if i need a timer circuit i am not going to build a 555 by smelting and refining sand i found on the beach , and then try to construct a wafer etc ... i buy a 555 , read the datasheet and off you go. If you are tasked to design a timer that delays 10 seconds you don't need to be able to calculate the weak nuclar bond...
there is such a thing as 'practicality'.

anyway. just my two cents of trying to help answer questions that people have about electronics, in an understandable manner. the world is already full of questons on how to calculate anLED series resistor and hard-whino 'programmers'. I'm trying to 'lift' it a bit beyond that ,without spooking people too much... We live in a wonderous universe. Let's go out and explore !

@dave : that is a well know 'alternate' explanation.
If you read mine attentively you will see that i never use the word 'base current'. It is the voltage difference ( potential) that causes the electrons to flow , and the electrons are charge carriers. it i indeed the pressure ( the 'voltage' , technical definition 'potential' that cases charge to move ) i control the direction it is allowed to flow by applying pressure ( or void thereof ) to the base. i will get electrons out of the base ( or in the base-e) because there is a conductive pathway ( something that is NOT there in a mosfet. There is a connection in a J-Fet though, but we polarize it in reverse to create the barrier that way. ).

if you look at transistor plots , some do specify the vbe/Ibe plot.. just like you can make an Ib/Ic plot you could make a Vbe/Ic plot ... ( substitute the Ib scale with the Vbe/Ic plot.. and you got it.)
it is all interconnected. you can explain it in voltage domain or in current domain. for bipolars it makes more sense to explain it in current domain... for a mosfet it works better in voltage domain.
pure practicality...

did you consider writing a book or teach at some schools? you would do fine

i always had problem in math, tons of unkown just letters without value or meaning, just solve that equation, what the hell for? it does nothing parctical the same equation with values or letters with assigned values were simple for me

#### free_electron

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##### Re: How transistors work
« Reply #15 on: May 06, 2012, 02:08:10 pm »
MOSFETs are even easier to explain at a qualitative level.
The Source to Drain path is a "channel" that can contain current carriers (as per FreeElectron's "doping" description of silicon.)  A voltage (charge) on the gate sets up an electric field (Thus "Field Effect Transistor") can move those current carriers into and out of the channel, just because "like charges repel and opposite charges attract."

Bingo ! You nailed it ! That's all there is to the working principle of a mosfet.

As for mathematics : they are just another modelling technique. One that can be very useful. But also one that can be very confusing. And, just like many modeling techniques is only as good as the detailedness of the model. Components today have so many properties that are still very hard to model or impractical. And nature doesn't give a hoot about the mathematics. You cannot go and yell at a component that it should behave as the numbers dictate.... That transistor will do as it pleases.. It is just a lump of 'stuff' where some interactions take place ... And you jump up and down and yell at it that it is not a 'nice transistor' because it doesn't follow your carefully calculated behaviour. If you are real lucky it will at least do something usefull. If you are unlucky it will violently commit suicide while jumping off the board and burying itself in the ceiling tiles, setting the rest of the building on fire...

I did a course at UCSC (university of california santa cruz) on signal integrity a while ago by one of the 'guru's in the field. He gave some examples using hyperlinx and some other tools and then he very quickly wrote that off. He gave an example to try to model crosstalk in a system. You have an agressor wire ( the wire carrying the'offending signal' that radiates energy ) and a susceptor wire ( the wire picking it up ).
These two wires run a complex path on a pcb with abunch of vias and layer hops.
They can extract the data from the real pcb layout , model it mathematically and run the numbers. Takes about a minute on a quadcore pc to get this nice color plot of the field strength and where the crosstalk happens...
Then the scenario changes... Three wires. Two agressors , one susceptor .. Calculation time : 30 minutes ...
Thee agressors one susceptor .... Just over 30 hours
Four agressors ?  Running on a Cray supercomputer ... 2 months and counting ... They had about half the plot ...
At that point the instructor said. From now on we drop all mathematics , and we switch to an 'instinctive' approach.?. If you have 4 signals blasting energy in a 5th one .. You are a bad pcb designer and should not be making boards. You were lost 3 signals ago. It is pointless. If you have 4 agressors you let it run out of hand.... So there is no point in even trying to model that...

Let me add a few tidbits of extra information.. ( i believe i posted these in another topic but i can't remember ... Anyway .. Here we go )
Current handling capability : for a given thickness : make the channel wider and you can send more current through it. Dope it harder and its resistance becomes lower.
Voltage blocking capability (vdsmax) make the gate wider. Just like you would make a moat wider to prevent barbarians from jumping over them.

Vgsth ( threshold voltage to switch the mos on. ) this is controlled by doping and layer thickness. The thicker the layer is the more voltage you need to get on the gate. Simply because the field needs to penetrate deeper in the channel. The threshold voltage denotes when the mos is conducting but you dont use the full 'depth' of the channel yet. So thinner mosfets have lower Vt

The mosfet gate is a big fat capacitor formed by the gate electrode and the channel. There is an isolation barrier there that acts as dielectricum.

Ah now i remeber which topics. It was about flas memories and analog multiplexers. I'll go and search and post the links below
« Last Edit: May 06, 2012, 02:34:03 pm by free_electron »
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#### A Hellene

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##### Re: How transistors work
« Reply #16 on: May 07, 2012, 12:55:02 pm »
This is an excellent opportunity to clarify what Vgsth is all about, since it is a common misconception that the moment Vgs becomes equal to Vgs_th, the FET becomes fully conductive; which is far from reality! Actually, when Vgs == Vgs_th, the MOSFET begins to conduct, with its drain current Id just becoming measurable (250µA at room temperature, typically). From this point there is a long way until the device becomes fully conductive and decrease its Rds_on to the advertised levels, which will allow the drain-source voltage drop Vds to become the minimum possible, decreasing the amounts of power dissipated on the FET.

Assuming we are dealing with enhanced-mode N-channel MOSFETs used as switches (being turned fully ON and fully OFF), this is a selective quotation of a message I have posted somewhere else:

Without making things more difficult to understand (by introducing the switching model of the power MOSFET or the dv/dt induced breakdown due to the parasitic bipolar transistor model), the transfer characteristics of a FET are based on the charge quantity accumulated to the gate capacitance, in order to "open" the Channel and start the drain current Id flowing; and vice versa, based on the charge quantity removed from gate capacitance in order for the Channel to "close" and stop the drain current flow.

Now, the parasitic gate capacitance is not as simple as it sounds to be. There are three distinct capacitances in a MOSFET: The Cgs (the Gate-to-Source capacitance), the Cds (the Drain-to-Source one) and, most importantly, the Cgd (the Gate-to-Drain capacitance). The data sheets define them as follows, since this way the capacitances can be directly measured:
1. The Input Capacitance: Ciss = Cgs + Cgd
2. The Output Capacitance: Coss = Cds + Cgd
3. The Reverse Transfer Capacitance: Crss = Cgd

Only the capacitance Cgs is linear; Cgd and Cds are voltage depended. Additionally, Cgd is the most important parameter of them all because it is the main feedback element between the input and the output of the device. It is the capacitance charged with large magnitude voltage (Vdg = Vds-Vgs) charges capacitively coupled to the gate and actively resisting any Vgs level change. Cgd is known as the reverse transfer capacitance or the Miller Capacitance. Please search for the Miller Effect. It is sufficient to be said that the Miller effect is what predominantly limits the device's switching speed; especially in high speed switching of high voltage loads, where the Miller effect becomes a very considerable factor.

Another important parameter given at the MOSFET data sheets is the Gate Charge Qg that can also be broken down into the distinct charges of Qgs and Qgd, where Qg = Qgs + Qgd and it is the minimum charge required to switch the device on. Defining the gate element charges helps calculations; for example, a 10nC charge can be moved to Cgs in 10msec time by applying a Gate current of 1mA, and so on. Knowing also that Q=C*V, I=C*(dv/dt), etc., it becomes easy to calculate the current, voltage and timing elements needed for the Gate drivers.

A FET responds instantaneously to changes of the gate voltage Vgs. There are four Vgs regions during the device turn-on period and the goal is to raise Vgs to the final value (Vgs_driver) as fast as possible:
1. 0V <= Vgs < Vgs_th:
The FET is off, while the rising Vgs is charging Cgs: The Id is minimum and equal to the leakage current of the device; Vds is maximum. This is the FET turn-on delay.
2. Vgs_th <= Vgs < Vgs_Miller:
The device begins to conduct. This is the FET linear region, when Id is proportional to Vgs and Vds has not substantially been changed from Vds_off yet; Id = g*(Vgs-Vgs_th), so Vgs_Miller = Vgs_th + (Id/g).
3. Vgs = Vgs_Miller:
This is the Miller Plateau region of Vgs, where Vgs remains constant until the gate driver has adequately charged Cgs and discharged Cgd, while Id has reached its maximum value and remains constant, and Vds begins to fall.
4. Vgs_Miller < Vgs <= Vgs_driver:
This is the last step of the FET turn-on, where Cgs and Cgd have been charged to the final point, and the final value of Vgs now defines Rds_on and, thus, Vds and the device power dissipation. (For example, the NTD60N02R parametric Rds_on characterisation is: "Rds_on=11.2mohm for Vgs=4.5V and Rds_on=8.2mohm for Vgs=10V")

This is the relation between Vgs and Vds versus Qg, from the NTD60N02R data sheets:

Vgs and Vds versus Qg.png

The turn-off procedure of the FET is the exact backtracking of the turn-on steps, above. The goal here is the same: To turn the device off as quickly as possible. Again, to turn the device completely off will need to completely discharge Cgs by pulling Vg to Vss, while the hardest part will again be overcoming the Miller effect, where Cgd will oppose any Vgs change, trying to keep Vgs at the Miller plateau voltage region.

-George

« Last Edit: May 07, 2012, 02:55:06 pm by A Hellene »
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)

#### free_electron

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##### Re: How transistors work
« Reply #17 on: May 07, 2012, 03:39:59 pm »
correct. here is how i explain this stuff :

think of the channel as a valve. the harder you press the more water will flow. when you press on the gate ( apply voltage) water will begin flowing but not at full rate. for that you need to press harder.

Vgsth is the moment where you succeed in getting noticable conduction between source and drain. but you are not 'pressing' hard enough to have the entire channel be open.
The gate is at the surface of the mosfet construction with the channel underneath. this channel has a thickness.
when you get the mos to conduct (Vdsth) you are only using the area right at the top of this channel. if you apply higher voltage you succeed in opening the 'deeper' area's untl you can use the full thickness of the channel.
a thin channel ( think of a thin wire) has more resistance than a thick channel ( thick wire ) . in essence the gate voltage controls how thick of a wire you have between source and drain.

so for switching you want to get the thick wire as soon as possible... after all ohms law dictates that current times resistance gives voltage.. and voltage times current is 'power' or ... heat!

you frequently hear about 'mosfet drivers'... what are thoe then you ask.
Well the limiting factor is this darned gate capacitance ( and the other two ) . a typical i/o pin of a processor can only deliver a few tens of milliampere... so you have a problem there to charge or discharge this gate capacitor fast. ( remeber that you want to go into full conduction , or come out of it, as fast as possible to avoid losses...  ) And your i/o pin has a maximum 3.3 or 5 volt swing.
This is what a mosfet driver solves. it is an amplifier that can deliver high peak currents and peak voltages. this can charge the gate very fast and discarge it very fast.

Now, you have to be careful with that.. charging and discharging a capacitor is displacement of electrons ... displacement of electrons is current! So there is actually an AC current flowing in the gate when you are driving the mosfet from , lets say , a PWM source...

This causes dissipation in the gate !  This ac current will also couple, through those parasitic capacitors... and you get all sorts of nasty effects ...
so , sometimes you need a small series resistor between such a gate driver and the real gate. ( typically 22 to 47 ohms ) to dampen this a bit.

For very fast transients there exist even special ferrite beads you can slide over the gate pin of the actual transisotr before you solder it down.
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#### A Hellene

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##### Re: How transistors work
« Reply #18 on: May 07, 2012, 05:21:44 pm »
This was a very nice analogy of the FET channel, as being a wire of variable thickness controlled by the gate!

Now, on the gate driving matter, I really regret I did not bother to capture the traces of the (rather fat) NTD60N02R gate in a low voltage (18V) SEPIC design, while I attempted to drive the FET directly by an AVR PWM output pin (with a true complimentary stage of Rds_on ~24ohm at Vdd=5V, for both the high and the low side output transistors) running at 64MHz. It was a surprise for me to see clearly at the gate trace the Miller plateau induced delay with Vgs clamped at Vgs_Miller, during both the gate charging and discharging phases! This was a real time Miller plateau visualisation, almost identical to the corresponding data sheets diagrams! In particular, when the FET load was connected Cgd was actively trying its best to oppose to any Vgs changes the AVR output line was struggling to make; with the load disconnected, thus with Cdg floating, I could only see the Cgs charging/discharging curves at the gate trace. Of course, adding a complementary emitter-follower stage gate driver using the BC639/BC640 matched pair and a 10ohn gate series resistor, the problem disappeared.

-George
« Last Edit: May 07, 2012, 06:01:38 pm by A Hellene »
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)

#### M. András

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##### Re: How transistors work
« Reply #19 on: May 07, 2012, 06:07:32 pm »
so if i want to pass 40 volts trough the mosfet i need a gate drive voltage above that 5-15 volts whats the treshold voltage? i dont really understand this aspect of the mosfet or its independent from the voltage what i want to pass trough it, the driver chips even the analog ones can provide this?

#### SeanB

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##### Re: How transistors work
« Reply #20 on: May 07, 2012, 06:30:54 pm »
No, at 5V it will start to conduct, and at 15v it will be fully on, irrespective of the current in the channel. If you turn it on to the point where it conducts with 40V across it the voltage on the gate will be somewhere between 5 and 15V. At 15V it will generally be full on, with around a volt or less across it, depending on current causing a voltage drop across the Rds on resistance. The 40V will be the breakdown voltage of the device when it is off, ie the voltage above where it will break down.

#### A Hellene

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##### Re: How transistors work
« Reply #21 on: May 07, 2012, 06:43:52 pm »
so if i want to pass 40 volts trough the mosfet i need a gate drive voltage above that 5-15 volts whats the treshold voltage? i dont really understand this aspect of the mosfet or its independent from the voltage what i want to pass trough it, the driver chips even the analog ones can provide this?

Through a FET (or any other component) only current (which is flow of electrons) can pass. That is because voltage is the reason of current flow. Or, in other words, without a voltage difference (an electrical potential difference) there cannot be any current flow. Wikipedia will adequately cover and explain in sufficient detail all the terms I have just used.

Now, for every single component available, the manufacturers supply their customers with special literature called the data sheets that contain every little detail about the use of their components. To answer your question, you will typically need to apply a signal of +10..12V magnitude to a MOSFET gate in order to turn the MOSFET fully on; in most of the cases, a MOSFET gate can withstand no more than ±20V before it is destroyed.

-George
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)

#### free_electron

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##### Re: How transistors work
« Reply #22 on: May 07, 2012, 08:16:32 pm »
here is the 'water' analogy :

'Voltage' is 'pressure'
'Current' is how many liters/second there flows.

For a given pipe thickness ( wire thickness / channel thickness ) and a fixed pressure you get x amount of water per second. increase the pressure (voltage) and the flow goes up (pipe thickness is constant).

The pipe thickness is resistance. The thicker the piper the lower the resistance....

Liters/ second = electrons / second.... electron/s = ampere ....
So a mosfet is actually a pipe where you control the thickness...
If you restrict the flow you create 'friction'... friction is heat ...

Now, there is a bit more about 'mosfets' that has not been told... in reality a mosfet does NOT have a drain or a source... It is the physical construction that 'creates' these.
A mosfet is really a resistor with a gate on top. just like a resistor has no designation for its electrodes , a mos does not have this.
All you need to do is 'lift' the gate 'x' amount of voltage above the channel. the current will automatically flow in the right direction.
In the physical mosfet there is actually a fourht terminal called the 'bulk'. we cannot leave that thing floating as there is a parasitic element that may pick up electrons and go conductive.. the mos would fail at that moment ( this 'parasite' is actually a bipolar transistor ). so we ( the people that make the mosfets ) tie this bulk to one end of the channel. this 'shorts' the b-e junction of the parasite and you are left with a diode. the location where the 'cathode' is connected we call the drain , the other one the source.

i will try to draw an a structural diagram that shows what is really happening there and how it is constructed. off to lunch now ...
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#### M. András

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##### Re: How transistors work
« Reply #23 on: May 07, 2012, 08:54:19 pm »
sorry if i wasnt clear, i know only current can flow and coltage is just potential difference between 2 points, so the fet gate is starts to turn on lets say at +4 volt applied to it relative to ground in the circuit and it will be fully on at +15, but not much over that it will be destroyed? if i understand it right there is no connection to the voltage of the fet operates at lets say its in a 40volts circuit switches some load, and the voltage must be applied to the gate to turn it on or off, but i can vary its resistance by adjusting the voltage applied to the gate, i gonna get some next time i somewhere near an electronic component shop, i would hate to destroy any of the innoncent things jsut to figure out how do they work and how to use them

#### free_electron

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##### Re: How transistors work
« Reply #24 on: May 07, 2012, 09:18:38 pm »
wait for the drawing. then it will become clear.
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).

Smf