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The capability of gallium nitride (GaN) high power transistors arises, in large part, due to piezoelectric polarizations that induce the formation of a carrier rich two-dimensional electron gas. These polarizations, in turn, are directly related to the strain and hence stress that is present within the transistor. As a consequence, the stress load, as well as its measurement, is extremely important to the optimization of this device class. In response, this study demonstrates a technique to quantify the magnitude of operational thermoelastic stress that evolves in a GaN transistor through simultaneous use of the Raman signal’s Stokes peak position and linewidth. After verifying the technique through comparison with a finite element model, the method is then utilized in the analysis of high electron mobility transistors grown on silicon (Si) and silicon carbide (SiC) substrates. For each series of device, the major stress contributors—thermoelastic, converse piezoelectric, and residual—are acquired and compared. While the magnitudes of the components are larger in those devices grown on silicon, the resultant biaxial loads in each of the devices are comparable at high power levels as the dominant residual tensile stress is counterbalanced by the compressive thermoelastic contribution.