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This paper presents a discussion on the substrate thermal design of backside-mounted power GaN high-electron mobility transistors. After a review on the thermal properties of the relevant materials and their temperature dependences, design guidelines are proposed on the basis of 3-D thermal simulations; the results presented suggest that in SiC-based devices, substrate thinning does not typically improve the thermal resistance or the dynamic thermal behavior. Contrary to what happens in III-V GaAs- or InP-based discrete or integrated devices, therefore, microstrip design on a thinned substrate (as opposed to coplanar design on a comparatively thick substrate) is generally not thermally superior. This should make possible, from the thermal standpoint, the realization of coplanar multifunctional GaN-based monolithic microwave integrated circuits integrating, e.g., low-noise and power stages and avoiding the use of via holes.