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I am just beginning to understand how electricity really works. I understand there is voltage, amps, and resistance. If electricity is water, voltage is like the pressure of the water and amps are the amount of water flowing through a specific point in a specific amount of time. Resistance is like the width of the water pipe.

I also know that electricity must flow in a "circuit." Basically, around and around. In other words, it has to have somewhere to go. If there is no circuit, there is no electricity.

I also know that a MOSFET transistor is "voltage controlled" not "amp controlled." I also believe that a MOSFET works a little bit differently than a Bipolar Junction Transistor because the electricity from the gate does not actually flow to the drain in a MOSFET because there is an insulator stopping it.

However, this makes me wonder about the fundamentals of a "circuit." From what I have seen in some explanations of online simulations of CPUs, such as this blog post from Ken Shirriff, the wire that goes to the gate of a MOSFET stops after it gets to the gate. This makes me think there is no "circuit" for the gate wire of a MOSFET since it just stops.

I can think of two possible explanations for this:

  1. If you connect a wire to the positive terminal of a battery, it has volts even though there are no amps flowing because there is no circuit. In other words, volts don't require flowing electricity to exist. This would explain why the gate wire of a MOSFET doesn't actually need a "circuit" to control the transistor.
  2. There is actually a circuit, it's just that the wire flowing to the gate of the MOSFET is actually just an offshoot of another wire that is completing a circuit.

Can you please help me clear up this misunderstanding.

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  • \$\begingroup\$ Hint. The region between the gate and the drain-to-source channel of the MOSFET is filled with an insulator. These would make a capacitor that can charge up to a voltage depending on the gate-source voltage. \$\endgroup\$ – dirac16 Jan 13 '17 at 19:17
  • \$\begingroup\$ The water pressure model is valud here as well. If your sink is turned off, there is still pressure in the pipes even though water is not traveling through them. \$\endgroup\$ – Cort Ammon Jan 13 '17 at 22:56
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    \$\begingroup\$ You may be interested in the term "electrostatics" en.wikipedia.org/wiki/Electrostatics and maybe the tag "electromagnetism" \$\endgroup\$ – user117772 Jan 13 '17 at 23:36
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Refer to my previous answer: Is voltage the speed of electrons?

Voltage is best thought of as a field. We're used to thinking of gravitational fields being entirely uniform, but magnetic fields are not. If you attach a piece of ferrous metal to a pole of a magnet, it extends the field into it. Similarly the electric field between the two poles of a voltage source can be extended with electrical conductors.

This extension of the field extends all the way to the field inside the field-effect transistor.

(Knowing about fields clears up a lot of misconceptions brought on by thinking about electrons. Ignore the electrons.)

Edit: so the explanation is almost exactly your (1) with one detail different. Dirac16's hint is important. The gate-to-channel insulation has a capacitance. So the circuit looks like a capacitor. These two circuits are equivalent (ignore the values):

schematic

simulate this circuit – Schematic created using CircuitLab

So there is no steady-state DC current flow, but at the point of connecting up the circuit the capacitor charges, and while it is charging a current flows. This is actually quite important when designing power MOSFET systems: you need to be able to supply adequate current for a very short time.

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  • \$\begingroup\$ Ok, that's a good place for me to start mentally. I appreciate you helping me approach this as a beginner. So, it sounds like the gate wires of a MOSFET may just be like the piece of ferrous metal attached to the pole of a magnet. They just extend that field out to the end of that gate wire and that's how the transistor switches states. But it does sound like the gate wires of a MOSFET require a current to produce the voltage. It's just that the current is flowing through the main part of the wire and not directly through that little offshoot wire going to the gate. \$\endgroup\$ – BarrettNashville Jan 13 '17 at 19:54
  • \$\begingroup\$ See edit - the MOSFET is a capacitor which requires a current to charge, which in turn requires a voltage to cause the current. \$\endgroup\$ – pjc50 Jan 13 '17 at 20:20
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Voltage can exist without a circuit. Or at least it can be difficult to see what the circuit is. For example, whenever you see a lightning strike, that's caused by a large potential difference (aka "voltage") between the ground and a cloud, or between two clouds.

In fact, that potential difference was developed as the wind took electrons from one place and deposited them somewhere else. But it's unlikely you'll ever figure out exactly where the electrons that charged a cloud negatively came from, or where the electrons that got stripped away to charge a cloud positively went to.

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A vacuum tube makes it a little easier to imagine (MOSFET.)

Think of two plates of metal in a vacuum. One is very hot and electrons are "boiled" off it it into a nearby gas of electrons. They stay near that hot plate because, having left it, it's now positively charged and they are attracted to it. But more boil off, so the cloud remains. In equilibrium, there will be an equal number of electrons boiling as re-attaching themselves back to the nearby hot plate of metal, but a small gas cloud of electrons will be present because of all that heating going on.

Now make the other plate very positively charged. The electrons in the "gas" will move towards the very positive plate and travel across the vacuum to get there. Normally, this would stop at some point because the hot plate will also become more positively charged as more electrons boil off and leave. Eventually, the whole thing just stops again. But so long as you add more electrons to the hot plate that is boiling off electrons, more electrons will be able to flow and there will be a continuous current. This is, in effect, a vacuum diode. If you place a voltage source across the two plates of metal so that the negative side is attached to the hot plate and the positive side is connected to the cold plate, electrons can keep on boiling off and more electrons will arrive to replace them. (Reversing that won't work, because almost no electrons boil off the cold plate. All that happens is that you pull the electron cloud back somewhat closer to the hot plate.)

Now, think of the vacuum diode in operation again with a current. You insert a metal screen (like a gridded screen door) between the original two plates and bring a third wire out for that. (Everything is still inside a vacuum, though.) The screen has such large holes in it that as the electrons travel across, almost all of them miss it and just go on through. A few might stick to it, but if so they will only add a little negative charge to the otherwise neutral surface (the electrons will stay on the surface of this screen because they also repel each other) and this negative charge will make it more unlikely that additional electrons will stick. Instead, they will more certainly go through those holes in the screen.

Suppose you now attach another battery, but this time with the positive side attached to the hot plate and the negative side to the screen. This will make the screen much more negative than the hot plate and it will "screen out" the ability of the electrons to "notice" the very positive cold plate on the other side. Negative enough, the electrons won't travel across the distance and will instead just stay next to the hot plate. But if you adjust this negative screen voltage downwards enough, then some of that very positive attraction will be noticed by some of the electrons that accidentally got further away from the hot plate than others and they will be able to avoid the negative screen and get through the holes and then be very much accelerated towards the plate again. However, it will be fewer, than if the screen weren't there at all.

So the screen can be used to "moderate" the flow of current between the hot plate (called a cathode) and the cold positive plate (called the anode.) The screen does this without any current of its own (it's repelling the electrons as it is negatively charged.)

Although the MOSFET details are quite different, this can give you an idea how a field, and only a field, can impact a current flow without actually appearing to be a full circuit loop of its own.

It does take a tiny "current" for a moment to charge the screen up, of course. But once a very few electrons are there, it's quite effective.


Another interesting note for you to consider. Why would an electron current within a wire follow the wire around a bend in it? Physically, there must be something that forces all those quadrillions of electrons flowing by to take the turn! It can be as little as one or two electrons sticking to the surface near that bend to cause all those electrons to turn!! Seriously! So when you bend a wire, and put a current through it, just one or two or three extra electrons will stick (in equilibrium, of course) to the surface of the bend and that is entirely enough added force to cause a huge torrent of electrons to curve around and bend with the wire.

It's pretty impressive when you think about it. Electric fields are very powerful.

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Crude rule of thumb: inductors "store current," while capacitors "store voltage." *1 A capacitor can maintain a voltage while sitting on a shelf, disconnected from any circuit.

So no, we don't need a circuit in order to have a voltage.

Or, the physics version: to create a constant electric potential, electric charge must always be present. A charged metal ball is surrounded by a radial e-field and a concentric array of equipotentials, the "voltage in space." No circuit is involved. Even a single electron or proton has a pattern of voltage in the surrounding region. Voltage is a way of measuring e-fields. (And, e-fields can exist without any electric circuits present. Only electric charge must be present.)

Cool animation of voltage-patterns surrounding charges during charge-separation. The lines are e-field flux, while voltage would appear as perpendicular lines running across the flux lines. Essentially this is an animation of a capacitor being charged, starting at zero.

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If there is no circuit, there is no electricity.

Actually, if there is no circuit, then "electricity" halts in place. (That's if the word 'electricity' is defined the way Maxwell/Faraday/Einstein and SI physics units defined it.) Metals are full of immense amounts of charge, of movable electrons. During electric current in copper wires, it's this "electricity" which must flow in a closed circle or "complete circuit." Beware of mistaking the motion of electricity as being a kind of electricity. Many grade-school science books make this mistake. Are they correct when they say that "electricity" is a flowing motion ...of electricity? What?! No! Or, whenever the electricity stops moving, does "electricity" disappear? Nope. Electricity, the charges, were already in the metal even before it was made into wires. It's correct to say that when electricity starts moving, "electric current" appears.

*1 Capacitors actually store energy in the form of e-field, with energy proportional to voltage squared.

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