I want to know why body of NMOS connected to ground instead of source in n-well process and creates body effect while PMOS is connected to source and is free from body effect?
Here is a cross-section of an n-well process, [source: wikipedia]
From the figure, we can see that the pmos is independent because each pmos is placed in separate n-wells. This is the reason why pmos transistors are often free from body effect because each n-well can be connected to the supply while the nmos transistors share the same substrate.