How Transistors Work [video]
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Today, field effect transistors (FETs) reign supreme for most IC applications such as CPUs and digital logic as they're more scalable and efficient than BJTs and have a very different structural design.
In fact, they invented a new part that has the "input" gate of an FET and the Collector-Emitter "output" of a BJT!
https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transis...
IGBTs are far from being a new invention.
And if we are speaking strictly, very high currents aren't switched with higher net efficiency by BJT than FET.
The reason SCR type devices are used for the kiloamperes range switches is due to them being the only switches which can mechanically/thermally handle so much current.
But for high voltages, bipolars will indeed go higher than FETs.
So for any BJT and FET we compare, passing a current less than V_ce_sat/R_ds_on is more efficient on the FET, and for greater currents the BJT is more efficient.
Depending on voltage. Extremely low RDs ON FETs are there. The real world choice would depend on whether you just need a constantly open switch, or high frequency switching for power conversion.
Are you in power electronics?
How semiconductor works: https://youtu.be/33vbFFFn04k
How a transistor works: https://youtu.be/DXvAlwMAxiA
I‘ve never been able to understand what the hell electricity actually is and how it works.
In general, though, the water analogy is actually quite good. The electron gas in conductors behaves a lot like a normal fluid. It easily covers linear components like resistors, inductors, and capacitors. Nonlinear components like transistors and diodes require more extensive analogy that is no longer very accurate.
The rest of the video seems pretty good though.
Did you invent this? It's fantastic. Lots of people misunderstand electricity as just "tiny little balls colliding with other balls", or think that the electrons themselves must be zooming around the circuit, rather than the wave they participate in.
"Electron gas" sounds right to my ears for these simple analogies. Just like sound oscillates, but still moves from speaker to ear, so too does AC current oscillate, but the energy has a single direction.
Sound behaves about the same if you entrap it inside a tube. The largest difference is that the electron gas is almost completely incompressible.
Electric charge has a magnitude (arbitrarily, we have labeled the axis such that protons are positively charged and electrons are negatively charged, but it's symmetric such that if everything in the universe swapped positive to negative nothing would change). Particles with opposite charges attract, particles with same charges repel. When a source of positive charge and a source of negative charge are separated, there is potential energy. We simplify our math a bit by factoring out the charge that would be moving from the positive to the negative out of our potential energy calculation to get "electric potential" or simply "voltage". When there is a path that charge can flow through between a high potential and low potential, it creates a flow of charge between positive and negative sources of charge that we call "current". As the charge flows, the potential energy decreases, meaning other energy has to be released. The most common way this happens is simply by creating heat. Some materials allow charge to flow through them more easily than others: the ratio of the potential (voltage) across a component to the rate of electric charge that flows through it as a result (current) is approximately constant for most things, and we call that resistance. This gives us Ohms law : V = IR
> I can’t explain that attraction in terms of anything else that’s familiar to you. For example, if we said the magnets attract like rubber bands, I would be cheating you. Because they’re not connected by rubber bands. I’d soon be in trouble. And secondly, if you were curious enough, you’d ask me why rubber bands tend to pull back together again, and I would end up explaining that in terms of electrical forces, which are the very things that I’m trying to use the rubber bands to explain. So I have cheated very badly, you see. So I am not going to be able to give you an answer to why magnets attract each other except to tell you that they do. And to tell you that that’s one of the elements in the world – there are electrical forces, magnetic forces, gravitational forces, and others, and those are some of the parts. If you were a student, I could go further. I could tell you that the magnetic forces are related to the electrical forces very intimately, that the relationship between the gravity forces and electrical forces remains unknown, and so on. But I really can’t do a good job, any job, of explaining magnetic force in terms of something else you’re more familiar with, because I don’t understand it in terms of anything else that you’re more familiar with.
A loop of marble and tube is static: the marbles all push against each other and are drawn to the tube.
If you pump some marbles up the tube, they’ll clump into other marbles, which will want to repel. They’ll scurry away, pushing the next barbles, and so on. That pump is a voltage, and the movement is current.
A resistor is a sludge that the marbles pass through. They can, but only if they’re being pushed by a voltage.
All matter is composed of squintillions of tiny things called "atoms", which are composed of a core having a certain positive integer (its atomic number) that represents its charge and a certain number of tinier things whizzing around the core called electrons. If the number of electrons whizzing around an atom's core is not the same as the atomic number, the atom gets mad and will either try to fob excess electrons off to surrounding atoms or steal them from surrounding atoms (depending on if it has more or fewer electrons than ideal).
Thus, if you decide to pick a few atoms and kick electrons out of them, it starts a chain reaction of electron motion that is observable on the macroscopic scale. These chain reaction is an electron current, which is measured as going in the opposite direction of the way electrons flow because Benjamin Franklin guessed wrong. The number of electrons flowing in unit time is the current (as a measurement), measured in amps (approximately 10 million trillion electrons per second).
It also turns out that you can vary how ferociously these electrons are hitting atoms: the voltage (amount of energy per ~10 million trillion electrons). Different materials are better or worse at absorbing (or resisting, if you will) the ferocity of electrons, and this is the resistance. If the resistance is high enough, it basically becomes impossible for the electrons to flow, and you get an insulator.
But how do you get the electrons to start moving in the first place? The easiest to explain involves chemical reactions: sometimes, atoms decide they'd rather be in a different orientation, and in the process of moving to that orientation, they need to emit some electrons first. With some cleverness, you can set things up so that electrons have to go around the "long way" (through a wire), and something that is set up to be able to do this is more commonly known as a "battery." The other main way you can do it is by creating changing magnetic fields, which are kind of created by changing electric currents, and explaining this in more detail basically requires throwing away everything I've described, starting from scratch with the actual physics, and still coming to the realization that mathematical equations are not satisfactory answers to the question "what is it."
He also explains seeing, heat, electro magnetism, elasticity and mirrors among other things.
His academic lectures are just as good but too long and hard for laymen to follow.
- Every time I switch on an electrical device I hear Feynman say 'Zzzzinggg' and I see the copper bars jiggling across town.
- Every time I see a cup of hot liquid I hear Feynman say "jiggling atoms"
- Watch his hands and fingers telling the more accurate science story, simulating the electrons and atoms.
- I would say that this the most important video to see for any human being on the planet. De second most important thing would be half of Alan Kay's lectures https://youtu.be/FvmTSpJU-Xc?t=2067
Great video's to watch with your kids! (from 3-4 years and older).
It's the other mechanism for charge carrier movement that causes the current through the base/collector depletion region: diffusion. This is really just thermal diffusion- the injected electrons will diffuse into an empty space in the same way that air quickly diffuses into an evacuated container. Once electrons have diffused into the collector / base depletion region, drift due to the collector potential "pulls" them the rest of the way.
Anyway, BJTs are partially a thermal device, since their operation depends on diffusion!
Physics Videos by Eugene Khutoryansky
Wes Hayward W7ZOI explains this side of things rather well in his book Experimental Methods in RF design. Some of the content is duplicated here discussing bipolar transistor feedback amplifier designs: http://w7zoi.net/transistor_models_and_the_fba.pdf
https://github.com/rhaido/200_transistor_circuits_united/blo...
Also he did a free book with fifty 555 circuits:
http://www.talkingelectronics.com/projects/50%20-%20555%20Ci...
And 100 IC circuits:
http://www.talkingelectronics.com/projects/100%20IC%20Circui...
In what way(s) do books tend to get it wrong, and is "The Art of Electronics" by Horowitz and Hill, which I'm currently reading, free of such errors?
The cardinal sin of teaching BJTs is to say that they're current-controlled amplifiers: little current in, big current out. They are not, not really. They're voltage-to-current converters: little voltage wiggle at Vbe, big current wiggle at Ic. This is really not in dispute and anyone who tries to argue for the hFE/beta model as really true is wrong. Certainly, it is a useful approximation and often all you need, but it is not how a BJT really works and so don't pretend it is.
(The link between the two is that the base is not a high impedance input. That is pretty uncommon, so it's not surprising that it's kind of confusing! Because it's a low-impedance input, small changes in applied Vbe correlate directly with small changes in current, so it looks current-controlled. But careful measurements, as I believe are plotted in AoE, reveal the primacy of the Vbe-controlled model.)
The one thing I do wish it would clarify though is that this is not a FET, and it's not discussing the kind of circuits used in digital logic. These are the transistors you se in analog amplifiers and very old computers.
Vbe = 0.7v, Ib = 1 mA, and Ic = 10 mA.
In practice, Vce might be 0.2v or so, depending on the transistor.
How can Vce < Vbe, when C-B-E essentially form a series circuit?
I liked this video, but I don't think this model can explain this observed behavior.
But it's way more complicated than that.
That is, the built-in potentials of the two diode junctions are in opposite directions, so in series combination those voltages mostly cancel out.
Since the base-emitter current heats up the junction, but the collector-emitter current doesn't (or rather it heats it up to the tune of 0.2 milliwatts per milliamp rather than 0.7) it stands to reason that this collector-base transition must be sucking heat out of the junction when the electrons cross it. And I think this is actually how thermoelectric generators (TEGs) and Peltier coolers made out of semiconductors like bismuth telluride work.
One problem with this account is that I think the band-bending diagram of a p–n junction has the electrons in the P material at a higher energy level, although I guess the relationship of the Fermi levels is opposite?
In particular, factors that have to be considered are the built-in potentials of the space charge layers (aka depletion zones), and the different doping profile of the collector versus emitter. The doping is very important, and is often neglected. But without it, transistors would basically operate equally well in forward mode as in reverse, and they definitely don't do that. These two things have a big effect on the Fermi/band-bend diagram and probably help clear up your last point.
I barely feel like I actually understand these things on a good day, and I pay the bills with them....
Just like QM itself, I guess. So, “shut up an calculate” all over again?
QM isn't too bad. The basic ideas are alien but straightforward enough. It's hard to extend small examples to larger systems, though, and that plus the innate weirdness really messes with your intuition.
BJTs are complicated devices built from multiple simple-enough effects all happening at the same time. Each piece isn't too bad, but you've got to track them all and combine them into an overall function.
QFT (quantum field theory, such as QED) is basically all the bad parts of both of those: the basics aren't too bad, but it's difficult to scale, there's a lot going on, and it's hard to develop intuition. QCD then makes the basic calculations intractable, which is why it's still poorly understood to this day (in my opinion).
So, yeah, in conclusion, complexity is complex? Not so profound, I guess.
Love seeing it in large scale as it becomes just another simple device like a resistor or potentiometer.
Transistors use a small current to control a larger current. You can think of this as one person tapping another on the shoulder of another person. But what good is that? Not much, on its own.
It is only possible to use this property to store data because you can build a circuit called a flip flop https://en.wikipedia.org/wiki/Flip-flop_(electronics) that enables 2 pulses of current to translate into 1 pulse of current.
That may not seem like much, but it enables everything happening in modern digital technology.
So how does it work?
Let's say you have 5 people in front of you, in a line. Everyone in line is directed to tap once on the next person for every 2 taps on their shoulder. So, for you, you feel 2 taps, and then you tap once. This is what a flip flop does.
Now, you get an additional instruction. When you are tapping with your left hand, you raise your right hand and when you are not tapping with your left hand, you lower your right hand.
Now, for every tap of the first person, the next person taps half as often, and the next person in line taps half as often, etc.
If everyone keeps time, looking at the group of people, you will see raised arms and lowered arms. The raised arms are 1s and the people without raised arms are 0s. Now you are counting in binary.
In a digital clock, this process is used to translate a quartz crystal's pulses into counting seconds and time. The same process can also be used to store numbers. For example, let's say I have 25 flip flops in series. Now, I can store a number as large as 2^25 in memory.
Going further, a mosfet doesn't use current to switch, it uses electric field. I like to say:
The charge that builds up on the gate from the applied voltage causes a depeletion(enhancement) region which is like Moses parting the red sea, so that holes/electrons can move through it.
So electrons-holes-electrons. No explanation for the asymmetric behaviour.
https://www.amazon.com/Code-Language-Computer-Hardware-Softw...
Thank you for the recommendation, it's a good one. I stand by my position that the video seems confusing without context. "Start with Why".
The analogy doesn't hold well for FET (field effect).
The last time I did it, it took me two hours, but I think that if you are coming at it without any previous knowledge of digital logic it should be tractable within about 40 hours.
Then the only remaining things to understand are:
① how can you create NAND gates with transistors?
② why are transistors better for this than, for example, relays, pneumatic valves, or neon tubes, all of which would also clearly work?
The answer to ① is actually pretty simple; the best explanation I've found is in The Art of Electronics, though Ken Shirriff's blog has a lot of great stuff on it too. And Falstad's circuit.js has some good simulations, like https://tinyurl.com/2eegeyxj and https://tinyurl.com/ybpo5ls2, which I think are nicely complementary to the theory.
I've never seen a good explanation of ②, but I think the main answer is that electrons are about ten thousand times lighter than atoms, so with the same energy, they move about 100 times as fast. The result of that is that switches that work by moving around electrons can switch a great deal faster than switches that work by moving around atoms. Neon tubes also work by moving around electrons, but they're slower than transistors for a different reason: it takes the gas a long time to thoroughly deionize.
Hard to find it all in one pdf most places on the internet but this blog has links to large amounts (maybe all, not sure) of it