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In this paper, a simple and accurate characterization method of the thermal impedance of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) is proposed. This method relies on low-frequency S-parameter measurements in the 100 Hz-3 GHz frequency range. It is shown that feedback hybrid parameter h12 provides an image of the thermal impedance in the frequency domain, which is independent of the size of the transistor. Very short thermal time constants involved in SiGe HBTs are accurately determined by this method, as well as the temperature dependence of the thermal impedance in a truly simple way as electrical measurements can be performed in dc conditions either on a wafer or on an attached die. Finally, a nonlinear electrical equivalent circuit model is extracted, which can be readily implemented in computer-aided design software for nonlinear simulation of any SiGe HBT circuit.