I am trying to make a variable resistor using a MOSFET, but the problem is that I don't want it to vary from voltage, but instead current. I understand that MOSFETs work by creating an electric field from the gate. I don't understand why the gate is not influenced by current.
Aren't electric fields influenced by current? If so, why aren't MOSFET gates increased with current?
All in all, my question is: why do MOSFET gates only change when you change voltage, and not current?
I suppose you could just tie the gate to ground with a resistor and inject a current. The current will be turned into a voltage that will appear across the gate. A resistor is a current to voltage converter after all.
simulate this circuit – Schematic created using CircuitLab
A MOSFET gate is essentially a capacitor that creates a field which allows current to flow through the body source and drain, and as the voltage increases, the body resistance decreases. The principle is similar to a vacuum tube, where grid voltage allows electrons to flow from the cathode to the positively charged plate.
Some current flows due to charging and discharging the gate capacitance, and some very small leakage current, but conductance is a function of gate voltage with respect to the source.
You can add a resistor from gate to source, and the current will provide a gate voltage and this will vary the resistance between drain and source, but it more a voltage controlled conductance.
MOSFETs can operate in a linear triode or ohmic mode, or in saturation or active mode:
First off I want to applaud you for asking a very interesting question, you're trying to connect the EM physics to the circuit theory and that's not easy and it's good youre thinking like this.
Electric fields are influenced by charge, not current. You might say current is moving charge, but in the DC case you mean it's a constant stationary current. Take a look at Maxwell's equations. It doesn't use current, it uses current density vector J, which is about charge.
Here's an analogy. Applying voltage is like taking a heavy box and lifting it onto a table. Current is like pushing the box across the floor. The MOSFET only cares about how many boxes it can see on the counter.
