[HN Gopher] How Transistors Work [video]
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How Transistors Work [video]
Author : simonebrunozzi
Score : 107 points
Date : 2022-09-09 17:06 UTC (5 hours ago)
(HTM) web link (www.youtube.com)
(TXT) w3m dump (www.youtube.com)
| Razengan wrote:
| OMG not another water analogy.
|
| I've never been able to understand what the hell electricity
| actually is and how it works.
| bhedgeoser wrote:
| This should help:
| https://www.youtube.com/watch?v=-wlw9U2k70o&ab_channel=Redst...
| akprasad wrote:
| I empathize, though it's a difficult problem:
|
| > 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.
|
| https://fs.blog/richard-feynman-on-why-questions/
| thehappypm wrote:
| Electricity is almost literally tubing filled with marbles that
| all violently repel one another, with the tube violently
| attracting the marbles.
|
| 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.
| wyager wrote:
| The initial analogy in this video is totally useless.
|
| 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.
| function_seven wrote:
| > "electron gas"
|
| 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.
| marcosdumay wrote:
| > 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.
| [deleted]
| deelowe wrote:
| I like the term field. It can be very intuitive if you have
| ever had a chance to interact with a Van de Graaff
| generator.
| marshray wrote:
| Unfortunately, a 'field' doesn't require electrons or
| holes, or provide a way to explain conduction.
| wyager wrote:
| Definitely not my invention. See also "fermi gas" which is
| a generalization.
| MoreSEMI wrote:
| The nearly free electron gas model is the standard
| introduction to explaining conduction in solids. In other
| words, no he didn't invent it. Interestingly enough, the
| first model for explaining conduction was naively assumed
| to be a simple gas model. After quantum mechanics was
| introduced into the modeling, it was discovered that while
| not quite correct, it was not very far off either.
| madengr wrote:
| morphle wrote:
| Richard Feynman is the best teacher we know (explaining
| electricity): https://www.youtube.com/watch?v=nYg6jzotiAc
|
| 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).
| jcranmer wrote:
| After some thought, here is my stab at explaining what
| electricity "actually" is, at least in a way that works well
| for classical modelling. (There is a necessary amount of
| handwaving and inaccuracies by omission, but I'm trying to keep
| it at an ELI5 level).
|
| 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."
| lxe wrote:
| You'll end up teetering between the math, which completely
| breaks down intuition about electricity, and analogies, which
| keep your understanding a bit grounded. Veritasium's videos on
| electricity were an attempt to de-metaphorize electricity, but
| they just yielded more questions.
| OkayPhysicist wrote:
| Electricity is the behavior of electric charge, and electric
| charge is an intrinsic property of some particles (it'd be like
| asking what mass _is_ , without falling back on what mass
| _does_ ).
|
| 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
| ur-whale wrote:
| Should be how a BJT (bipolar junction transistor) works.
|
| The analogy doesn't hold well for FET (field effect).
| aerlinger wrote:
| This video covers a particular type of transistor known as the
| Bipolar Junction Transistors (BJTs). These are more commonly used
| in analog applications like amplification and signal processing
| rather than digital logic (though they can be used in specialized
| digital logic circuits).
|
| 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.
| marshray wrote:
| BJT's are also still used for some very high-power applications
| because they can be more efficient at very high currents.
|
| 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...
| FunkyDuckk wrote:
| FETs are also more common for many analog applications.
| lawrenceyan wrote:
| And FinFETs are state of the art currently for semiconductor
| applications.
| adgjlsfhk1 wrote:
| technically gate all around is sota, but not yet in
| production
| commandlinefan wrote:
| Any videos on how FET's work?
| spullara wrote:
| https://www.youtube.com/watch?v=c-3p8moNXfI
| amelius wrote:
| It would be great if someone could turn this into an online
| simulation.
| gw99 wrote:
| As always these things tell us how transistors work, which is
| fine and all that, but that's not enough to tell anyone how to
| use one which is almost universally done incorrectly even in some
| high profile textbooks. They are a little more complicated than
| this video can muster. In fact I'd argue that it's probably worth
| skipping this and delving into the mathematics properly.
|
| 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
| bergenty wrote:
| Interesting, I always thought it didn't make a difference how
| much current was applied to the emitter, that it was basically a
| binary switch above some very small threshold.
| westcort wrote:
| Here is how I explain it.
|
| 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.
| sbf501 wrote:
| Nice. But that's a BJT in saturation or cutoff. Which is only
| part of the story. The magic of a BJT is in quiescent region,
| which is like using a crowbar to lift something really heavy:
| the lever amplifies your force the same way a little base
| current amplifies the Emitter/Collector current.
|
| 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.
| mhh__ wrote:
| Bob Widlar's notes on this are really good
| lxe wrote:
| The entire channel is worth subscribing to.
| qwertox wrote:
| Does anyone have a good recommendation for a lecture on
| transistors? Like from Stanford or MIT?
| marshray wrote:
| http://amasci.com/amateur/transis.html
| unethical_ban wrote:
| He uses water, which I think tries to touch on the utility of a
| transistor and semiconductors. The video should have ended or
| been followed up with the reason any of what he described matters
| - how transistors are used in computing.
| lisper wrote:
| There's a book for that:
|
| https://www.amazon.com/Code-Language-Computer-Hardware-Softw...
| unethical_ban wrote:
| I own both editions and am re-reading with 2e.
|
| 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".
| lisper wrote:
| It's hard for me to imagine anyone with internet access not
| knowing why transistors are important.
| ceronman wrote:
| This is a good explanation, but I prefer the one by Ben Eater:
|
| How semiconductor works: https://youtu.be/33vbFFFn04k
|
| How a transistor works: https://youtu.be/DXvAlwMAxiA
| goalieca wrote:
| I actually studied with Britney as my mentor in 2nd year EE
| https://britneyspears.ac/lasers.htm
| mibsl wrote:
| Semiconductors - Physics inside Transistors and Diodes
|
| Physics Videos by Eugene Khutoryansky
|
| https://www.youtube.com/watch?v=hrpPKCDLRN0
| marshray wrote:
| Say an NPN BJT is in conduction mode with:
|
| 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.
| gw99 wrote:
| Using that simplified model, it's better to think of it as a
| simple diode between B-E and a completely separate current
| source between C-E.
|
| But it's way more complicated than that.
| ajross wrote:
| I like this one. I like especially that it stops to explain the
| junction bias volage and why it becomes the "diode drop" (even if
| it doesn't get into the implications), which is IMHO something
| most new people struggle with when understanding analog circuit
| behavior. Likewise it's careful to explain that the BJT
| transistor behavior is due to careful tuning of dopant levels,
| something that took me a long time to grok (FETs are easier to
| understand).
|
| 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.
| xor99 wrote:
| I have always found it really useful to go back to the first few
| examples of a device when learning about its function (e.g.
| https://www.extremetech.com/extreme/175004-the-genesis-of-th...).
|
| Love seeing it in large scale as it becomes just another simple
| device like a resistor or potentiometer.
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