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The increasing data rates demanded by third generation cellular communication systems and other high frequency applications require the use of power amplifiers operating at frequencies exceeding 1 GHz with output of the order of hundreds to thousands of watts. The area of very high frequency, very high power electronics is currently dominated by vacuum tube based devices as conventional semiconductor devices suffer from relatively low breakdown voltages precluding their operation at very high voltages and high powers. The vacuum tube based devices, however, suffer from the issues of high cost, large size and reliability issues. In recent years AlGaN/GaN HEMTs have demonstrated output power densities as high as 11 W/mm operating at microwave frequencies greater than 10 GHz. The extremely high output power density levels are achieved due to the high breakdown voltages of these wide bandgap devices and due to the large polarization induced charge leading to high output current densities. This work investigates the performance of the AlGaN/GaN HEMT using device modeling. Polarization effects have been incorporated using a highly doped AlGaN spacer layer. This thesis examines the effect of the device structure and doping profile on the AlGaN/GaN HEMT’s microwave performance including the unilateral power gain and maximum frequency of oscillation.