MOSFET inverter

Do you want to model the mosfet characteristics or just the inverter? If it's just the inverter you can model the mosfet as a perfect switch and use the square () function. This will model the inverter as a voltage source. Then you can add the output impedance of your fets and model it as a thevenin equivalent. If you want to model the mosfets, your simulation will be much more complicated. But it can still be done.