AlGaN - GaN heterostructure field effect transistors (HFETs), while capable of delivering large output power densities at high efficiency into the mm W range of frequencies, also dissipate high power densities in the form of heat. Elevated channel temperatures reduce device performance and raise reliability concerns. Understanding the temperature and power dissipation dependence of the thermal resistance of GaN HFETs is also important for accelerated life testing. This work employs an accurate channel temperature measurement technique to extract the thermal resistance of the GaN device structures. Three dimensional numerical modeling using temperature dependent thermal conductivities was also performed. Simulation results agree well with measured thermal resistance. Further simulations were used to design low thermal resistance device structures which exploit the high thermal conductivity properties of diamond. (c) 2007 Elsevier Ltd. All rights reserved.