Try to drive it with a square wave. 0v for off and +13v saturated on.. this voltage is for the gate REFERENCED to the source.... nothing else matters to it, only the voltage between the source and the gate.
If you look at how a field effect device works you will see why this is.
Long story short. We have a NPN structure. We'll call the first N doped silicon the source, the P doped one next to it (is isolated for this story), and then the last N doped silicon is our drain.
If we put our source to ground, and our drain to a positive... the electrons can go from the left N into the doped P but get stopped at the next P-N barrier (reversed biased diode really).
So nothing happens.
Now we place an insulated layer on top of the NPN silicon matrix... then a conductor that sits on that.... thats the gate. It is not connected to anything else at this stage.... so still nothing happens.
Now if we place a potential between the insulated gate +, and the source, we create an electric field. This effects the P layer (pushes the positive carriers away, creating a N inversion layer), and basically turns it into a N substrate.... now we have a NNN from source to drain, acting as a single block of N type material... the more positive .....the more conductive the new "N" layer so the resistance is all gone... it's turned on.
If we now take away the gate charge and drive it back to the same potential as the source... it turns back into a reversed biased diode... and the channel no longer conducts.
Do you now see why you MUST reference the gate to the source. You cant generate the electric field above the matrix and turn the P into N unless it is with respect to the source.... nor can you turn it off again unless you bring it back to source potential.
Note the gate is insulated from the other substrate, which gives us a very high impedance input.
Beyond +-20v Vgs, will puncture the isolation to the gate, 3 volts is enough electric field to start turning the N channel on, and conduction begins. We need to turn it up to 12-15v to for enough electric field to make darn sure it is fully on.
You do not want a partially turned on source drain path. You want it off or on..... nothing in between.
At least thats how I remember it. There are probably a few more things to worry about (body diode, capacitance etc), But thats the gusts of it I think.
Someone else can correct any blues.
................oztules
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