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In this letter, the self-heating effects in SiGe heterojunction bipolar transistors (HBTs) and the influence of the emitter width WE and the emitter length LE on all of the thermal parameters are presented for the first time by extracting the thermal resistance RTH, capacitance CTH, and time constant TTH. First, we discuss the thermal impedance extraction method. The approach is based on both dc and ac measurements within the 30 kHz-6 GHz low-frequency range. Then, the self-heating influences of various SiGe HBT topologies are presented. Finally, RTH, CTH, and TTH, which were extracted from different transistor geometries, are compared.