We investigate the influence of the device geometry of large scale RF power AlGaN/GaN-HEMTs on the RF output power density. Multifinger-RF-HEMTs with different device geometries were fabricated on comparable epilayers grown on both sapphire and SiC substrates. Thermal impedances of the different FETs were measured and simulated. We discuss the correlation between the thermal device impedance and the RF output power, measured at 2 GHz. Degradation phenomena under high temperature and high current conditions are reported. As a result, we show that the thermal device impedance can be used as a design rule for the optimization of AlGaN/GaN-RF-HEMTs. (C) 2002 Published by Elsevier Science Ltd.