SOI enables reduced capacitive coupling in RF power technology but the thick oxide causes thermal problems. In this paper, the authors present a new type of substrate, where the oxide insulator and the silicon substrate in SOI, are replaced by silicon carbide (SiC). SiC has higher thermal conductivity and is semi-insulating which can improve the thermal and RF performance. Here, LDMOS-transistors are processed and characterized on 150 mm silicon-on-polycrystalline-silicon carbide (Si-on-poly-SiC) substrates as well as on high power and RF optimized SOI reference substrates. The electrical performance for the Si-on-poly-SiC was improved or equal compared to the SOI reference and the device self-heating was reduced. The hybrid substrate had lower RF losses and the RF measurements on transistors were not ideal due to no isolation between the devices. (C) 2011 Elsevier Ltd. All rights reserved.