This thesis describes the research activities that have been investigated for improving the 3rd order intermodulation distortion products (IM3) and power added efficiency (PAE) and bandwidth performance of microwave GaAs MESFET power amplifiers. Two novel circuit techniques, one for improving the 3dB bandwidth performance and the other for improving the IM3 and PAE performance, were proposed and verified through simulation and practical measurements. The technique of including lumped elements matching networks within the package encapsulation (Close-to-Chip lumped element matching) of a 2GHz MESFET device is described for the first time. Simulation results showed that the amplifier using this technique had a 3dB bandwidth 3 times wider than the amplifier with Off-Chip distributed element matching. The linearity and efficiency performance of a 2GHz MESFET was improved significantly by presenting a difference frequency shunt short-circuit termination across the drain terminal. A 16dB reduction in IM3 and an improvement of 4% in PAE performance was measured on the bench. Success with this technique was further demonstrated with digitally modulated signals.