The effect of device design on maximum channel temperature for AlGaN/GaN high electron mobility transistors was investigated through finite element thermal simulations for dissipated power densities up to 5 W mm-1. The effects of substrate material, die size, and number of gate fingers were examined, as well as the divergence between maximum channel temperatures for two dimensional and three dimensional simulations. The maximum temperature increased as the die size decreased beyond a critical distance from the gates. The critical distance was dependent on the substrate material, with SiC requiring the smallest critical distance. At the maximum dissipated power, the temperature increased above ambient more than 300 °C for devices on sapphire substrates compared to an increase of 44 °C for devices on SiC substrates. As the number of gate fingers increased, the maximum channel temperature also increased, and a temperature gradient was observed along the gate width as well as between gate fingers.