From the physical level of the device to the circuit level

Formation of inversion layer

Vg rises to form a depletion layer.

When the capacitance of the gate oxide layer and the capacitance of the depletion region are full and kept constant, the inversion layer begins to form.

Channel charge increases.

P+ heavy doping to reduce threshold voltage.
In practice, the "intrinsic" threshold voltage obtained by the above formula may not be suitable for circuit design, for example, VTH=0, so the device will not turn off when VG=0.Therefore, in the process of device fabrication, the threshold voltage is usually adjusted by injecting impurities into the channel region, which essentially changes the doping concentration of the substrate near the bottom interface of the oxidized p-type village.For example, if a thin p* layer is formed, as shown, the gate voltage needs to be increased to deplete this region of the p+ dopant used to change the threshold voltage.

Uneven distribution of channel charge

Beyond the threshold voltage, the on-resistance gradually decreases as the gate voltage increases.

Channel charge

1, Linear Area

2, channel saturation region