Using accurate MOSFET substrate parameters obtained by a RF direct extraction method, it is demonstrated that a BSIM4 model with only substrate resistances is not physically valid to apply in the wide range of gate length because of scaling inaccuracy. In order to remove the unphysical problem of the BSIM4, a modified BSIM4 model with additional dielectric substrate capacitance is used and its physical validity is verified by observing excellent gate length scalability.