I am having lots of trouble trying to understand how the mosfet is triggered. The text I read assumes the source of the NMOS connect to ground, while a positive voltage is applied at the gate.
Because the source is grounded, VGS (voltage between gate and the source) = VG (voltage of the gate). And then, it claims that because a higher VG accumulates positive charge at the surface of the gate, which repel the holes beneath the oxide layer, leaving the negative ions. So the charge density beneath the oxide layer is:
where Qd is the charge density at the depletion region created by applying VG.
Now this is my question: WHY does the term VGS instead of VG? If, say the source is not grounded, but connect to something like 0.01v, then VGS is not equal to VG, and to overcome VTH (threshold voltage), I didn't see why we need to consider the source voltage To me, this doesn't make sense because repelling holes and creating the depletion region beneath the oxide layer has nothing to do with the voltage level at the source; only the gate voltage matters. Why VGS then?