nMOS ENHANCEMENT mode is when you aply positive voltage at the gate and it form a channel under the gate ...so it attracts the electronwhereas when u apply negative voltage ta the gate , it actually deplete the region under gate bacoz of negative supply...
For analog design circuit, u hav to...
U have to bias the MOSFET to have the full output swings. choose your bias point so that in any case it should not go out of saturation.
the equation for mosfet in saturation region is
Id=1/2 U Cox W/L(Vgs - Vt)^2
where U = mobility, Cox= capacitance and W/L = width and legth of mosfet...