How do I treat a PMOS compared to an NMOS in small-signal analysis?

Question

What exhaustive list can I use to determine the differences between NMOS and PMOS devices when doing small-signal analysis?

Equivalently, what are the fundamental differences between NMOS and PMOS devices, and how do related terms (i.e. treshold voltage, which is either positive in NMOS and negative in PMOS, or positive in both but with reversed polarity in PMOS, affecting many formulas) differ between them?

Answer 1

The only real difference is in the mobility of the carriers, where electrons are faster than holes (around 2x) thus giving nMOS transistor a 2 times better performance for everything else equal.

About notation, it's mostly a practical thing, and the small signal model is an abstraction that you make to analyze a circuit. So you can use it the way you understand it better.

For instance, I usually use magnitudes (flipping notations drives me crazy) and then determine the direction of the current and voltage from the circuit. So the nMOS will generally have a current from drain to source, and the pMOS from source to drain, both with positive sign.

pMOS's \$I_{SD} \$ will be proportional to \$V_{SG} - |V_T| \$, where nMOS's \$I_{DS} \$ will be proportional to \$V_{GS} - |V_T| \$. Note that for the pMOS, you can flip SG and SD and still obtain the right values, as long as you use the absolute value of Vt.