A methodology has been proposed to accurately model the gate stack of Gallium Nitride (GaN) based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). Small-signal analysis has been performed for the device biased in the spill-over region, where electrons accumulate at the insulator/III-Nitride 'critical' interface. Accounting for the barrier layer resistance with an accurate model yields a near but inexact match between simulation and measurement. It is found that the border trap admittance and the energy distribution of the interface trap capture cross section (σ) need to be included in order to achieve a very close one. It is concluded that the interface trap density, extracted using conventional small-signal admittance methods, can be significantly off if these non-ideal effects are not incorporated within the equivalent circuit models of GaN-based MIS-HEMTs. © 2013 IEEE.