SiC MESFET and III-N HEMT based on SiC substrates are the most promising structures for the next generation of RF power devices. One of the most useful material properties of SiC substrate is its high thermal conductivity, 3 times that of Silicon. But still, due to extremely high power density dissipated in this devices, an effective thermal management will be the key for future development. The main other requirement put on SiC substrate for SiC MESFET applications is purity, for minimizing carrier trapping, but also for thermal issues. For III-N HEMT, the impact of SiC substrate purity on the device has hardly been studied yet. Improvement in bulk growth techniques has led to larger diameter and purer substrates - though not cheaper yet - but many crystal defects, still present in the SiC substrates, are detrimental to large devices for both SiC MESFET and III-N HEMT.