Electronic – Does voltage require a circuit

mosfetvoltage

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.

Best Answer

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.