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We studied a temperature increase and a heat transfer into a substrate in a pulsed operation of 0.5 length and 150 μm gate width AlGaN/GaN HEMTs grown on silicon. A new transient electrical characterization method is described. In combination with an optical transient interferometric mapping technique and two-dimensional thermal modeling, these methods determine the device thermal resistance to be ∼70 K/W after 400 ns from the start of a pulse. We also localized the high-electron mobility transistor heat source experimentally and we extracted a thermal boundary resistance at the silicon-nitride interface of about ∼7×10-8 m2K/W. Thermal coupling at this interface may substantially influence the device thermal resistance.