In this paper, we propose an accurate method to extract the free carrier mobility in FETs. This derivation relies only on the drift-diffusion transport model without the need to predefine the gate-voltage-mobility dependence. This approach has been assessed with TCAD simulations, confirming its robustness even in the presence of short channel effects. Exploiting the proposed model, free carrier mobility is obtained in a 28-nm commercial bulk CMOS process at room temperature.