I am trying to design and build a basic signal detector using MOSFETs because they are more compact than resistors, capacitors, and inductors. I was wondering if anyone here has any suggestions on how I can do this project. The original thought I had was to simply look up a simple Field Strength Meter (attached below) and simply replace the capacitors in the circuit with properly biased MOSFETs so that they can be used effectively as capacitors. This approach seems too naive, if anyone has a more refined approach please feel free to tell me.
So first of all, using a semiconductor as adjustable capacitance is pretty "normal" – it's typically PIN diodes, though, which you can DC-bias to give them a specific junction capacitance.
PIN diodes are "bad" rectifiers, as they have high forward resistance – the "I" in PIN is for "intrinsic", and that means undoped semiconductor, which means good isolator.
Now, if you're really doing semiconductors, then you'll inherently have non-linear behaviour. Your crystal radio (which your "tuned antenna->rectifier" setup is, in fact) will hence have a nonlinear I/U line – which means it could be used as a mixer :)
So, since it'll be very hard to get things sufficiently exact, and not too lose all accuracy, stability and signal in the inductance, I'd rather go for a simple super-heterodyne receiver in this situation.