The AlGaN/GaN high-electron-mobility transistors (HEMTs) fabrication and its dc characteristics have been performed on both 4-in. diameter Si and sapphire substrates. Due to the high crystalline quality with one order low dislocation density of GaN on sapphire substrate, better 2DEG mobility with high values of drain current density (511 mA/mm) and extrinsic transconductance (198 mS/mm) were observed. Large OFF-state breakdown voltage (BVgd) with low gate leakage current was observed on sapphire-based HEMTs. Though the sapphire-based HEMTs show better characteristics than Si-based HEMTs, the thermal dissipation from the device is not good enough. The low thermal resistance of Si-based HEMTs leads the existence of high ON-state BVgd. This has been confirmed by measuring the device surface temperature (T-D) by infrared microscope camera. Higher value of T-D = 74 degrees C at V-DS = 20 V and V-GS = +1.5 V was observed on sapphire-based single finger 400-mu m-wide and 2- mu m-gate-length HEMTs when compared to the HEMTs on Si. (T-D = 41 degrees C). The measured T-D has been directly correlated with the measured drain current reduction (I-Dreduc.) due to self-heating. About 83% of high rate of increase in temperature by self-heating was observed on sapphire-based HEMTs when compared to Si-based HEMTs. The device thermal impedance (R-th) of Si- and sapphire-based AlGaN/GaN HEMTs are 44 and 139 degrees C/W, respectively. The ratio of R-thsi/R-thSapp = 0.30 obtained from our studies is consistent with the reported values of 0.297. The 4-in. diameter Si has been shown to be a viable alternative substrate for low-cost high-power operating GaN HEMTs. (c) 2005 Elsevier Ltd. All rights reserved.