Thermal design of gallium arsenide MESFETs for microwave power amplifiers

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In the traditional design of microwave power amplifiers using gallium arsenide field effect transistors, the active devices are mounted onto a thermally conducting heatsink, and matching circuits at the input and output take the form of microstrip, usually using aluminium oxide as the dielectric. The thermal attachment of the transistor is often a problem with this technology and the use of many bond wires can lead to manufacturing problems because of unpredictable phase shifts associated with their variable length. The author describes a number of surface mount alternatives to this technology and assesses the associated thermal implications. Three dielectric alternatives are considered, namely aluminium oxide, aluminium nitride and diamond.