Self-heating is an important issue for GaN heterostructure field effect transistors (HFETs), especially in high power applications. Here we report the temperature dependence of the dc characteristics of some GaN HFETs including the variation of the transconductance. We present the characteristics as a function of added power, instead of voltage bias, and use the temperature data to transform the power dependence into a dependence on the average device temperature. For similar devices on sapphire and SiC, at 20 V V-DS and 0 V V-G, the temperature increase for the same added power is similar to 2.7 times greater in the sapphire-based device. (c) 2006 American Vacuum Society.