DC I-V output,small signal and an extensive large signal characterization(load-pull measurements) of a GaN HEMT on a SiC substrate with different gate widths of 100μm and 1 mm have been carried out.From the small signal data,it has been found that the cutoff frequencies increase with gate width varying from 100μm to 1 mm,owing to the reduced contribution of the parasitic effect.The devices investigated with different gate widths are enough to work in the C band and X band.The large signal measurements include the load-pull measurements and power sweep measurements at the C band(5.5 GHz) and X band(8 GHz).When biasing the gate voltage in class AB and selecting the source impedance,the optimum load impedances seen from the device for output power and PAE were localized in the load-pull map.The results of a power sweep at an 8 GHz biased various drain voltage demonstrate that a GaN HEMT on a SiC substrate has good thermal conductivity and a high breakdown voltage,and the CW power density of 10.16 W/mm was obtained.From the results of the power sweep measurement at 5.5 GHz with different gate widths, the actual scaling rules and heat effect on the large periphery device were analyzed,although the effects are not serious. The measurement results and analyses prove that a GaN HEMT on a SiC substrate is an ideal candidate for high-power amplifier design. DC IV output, small signal and an extensive large signal characterization (load-pull measurements) of a GaN HEMT on a SiC substrate with different gate widths of 100 μm and 1 mm have been carried out. Flash the small signal data, it has been found that the cutoff frequencies increase with gate width varying from 100 μm to 1 mm, due to the reduced contribution of the parasitic effect. the devices investigated with the different gate widths are enough to work in the C band and X band. load-pull measurements and power sweep measurements at the C band (5.5 GHz) and X band (8 GHz) .When biasing the gate voltage in class AB and selecting the source impedance, the optimum load impedances seen from the device for output power and PAE were localized in the load-pull map. The results of a power sweep at 8 GHz biased various drain voltage demonstrate that a GaN HEMT on a SiC substrate has good thermal conductivity and a high breakdown voltage, and the CW power densit y of 10.16 W / mm was obtained. The results of the power sweep measurement at 5.5 GHz with different gate widths, the actual scaling rules and heat effect on the large periphery device were analyzed, although the effects are not serious. The measurement results and analyzes prove that a GaN HEMT on a SiC substrate is an ideal candidate for high-power amplifier design.