Why nMOS transistor is selected as pull down transistor?

• A static CMOS gate has an nMOS pull-down network to connect the output to 0 (GND).

• An NMOS device pulls the output all the way down to GND, while a PMOS lowers the output no further than |VTp| — the PMOS turns off at that point, and stops contributing discharge current. NMOS transistors are hence the preferred devices in the PDN.

• NMOS device fails to raise the output above VDD-VTn .