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This reported work deals with an accurate characterisation method dedicated to the determination of the thermal impedance of gallium nitride based high electron mobility transistors (GaN HEMTs). The method is inspired by the `3ω method` initially proposed by Cahill (reference 8 of this Letter) in order to measure the thermal conductivity of bulk materials or layers and the present authors` previous works on thermal resistance measurements. It is demonstrated that the voltage oscillation at the third harmonic is a real image of the thermal impedance of the device in the frequency domain. Both the theoretical approach and the test bench are discussed.