Carrier mobility
Beginner
How easily electricity moves through the material of a switch. Higher mobility means a stronger, faster switch.
Novice
How fast electrons or holes drift for a given electric field. In silicon, electrons (which carry NMOS current) are more mobile than holes (PMOS), so a PMOS has to be wider to carry the same current.
Expert
sets the PMOS width needed to match NMOS drive: roughly 2–3 in classic planar processes. The ratio is process-dependent, so the preferred P/N width ratio differs from kit to kit.