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We report on the noninvasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN HFETs grown on sapphire and SiC substrates. Micro-Raman spectroscopy was used to produce temperature maps with /spl ap/1 μm spatial resolution and a temperature accuracy of better than 10/spl deg/C. Significant temperature rises up to 180/spl deg/C were measured in the device gate-drain opening. Results from a three-dimensional (3-D) heat dissipation model are in reasonably good agreement with the experimental data. Comparison of devices fabricated on sapphire and SiC substrates indicated that the SiC substrate devices had /spl sim/5 times lower thermal resistance.