Electronic – How a Cmos not gate works

M2 is essentially acting like a pullup resistor in this case. Real resistors are difficult to make on silicon chips, so a PFET in on-state is good enough for this purpose.

The chip designer can vary parameters like the channel length, width, and possibly doping level. Depending on the characteristics of the transistor, it could act more like a current source than a resistor at the operating point. Sometimes a "long tail FET" is used to make a rough current source. Without knowing the parameters of M2, we don't know if it is more like a resistor or more like a current source, although in this application that wouldn't make much of a difference. Ideally you'd want a current source for a pullup, but lots and lots of places you see resistors doing that job well enough.

No, in CMOS both, the N-channel and the P-channel MOSFET are enhancement types.

The N-channel is enhanced by positive voltage at the gate with respect to its source.
The P-channel, however, is enhanced by negative voltage at the gate with respect to its source.

Thus

• a H at the input turns on the N-channel MOSFET (positive voltage between its gate and source) and turns off the P-channel MOSFET (0V between its gate and source) and
• a L at the input turns off the N-channel MOSFET (0V between its gate and source) and turns on the P-channel MOSFET (negative voltage between its gate and source).