how a field effect transistor works

regular FET pinches off (depletion mode)

has input impedance around 1 megohm

MOSFET (metal oxide semiconductor),
also known as IGFET (insulated gate),
has a layer of insulation above a transistor junction.

very high input impedance, around 1E12 ohm.

most mosfets are enhancement mode (naturally off).

when MOSFET is off, electrons flow from N to P, and then are stopped by a reverse biased junction, from P to N.

voltage on gate creates electric field, causing electrons to migrate toward gate.

electron density sufficiently high that the P material changes to N material in the neighbourhood of gate, creating an N channel.

the N channel means the device is on, and on fully (no junction drop).

true ohmic connection (unlike current mode of BJT)

digression into how a transistor works, depletion mode FET, IGFET (MOSFET), etc.,
historical context: diode, triode, ...