The aim of this paper is to present a detailed and thorough investigation of the dips in the magnitude of the impedance parameters for AlGaAs/GaAs HEMTs. These dips should be attributed to the resonance between the extrinsic inductances and the intrinsic capacitances. As a consequence, the resonance frequencies associated to these dips can be analyzed to determine the intrinsic capacitances when the extrinsic inductances are known. To verify the validity of this straightforward approach, the extracted capacitances are compared with the results obtained with the conventional modelling method. The comparison is performed versus the gate source voltage and the gate width.