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This Small Business Innovation Research (SBIR) Phase I project comprises the design, fabrication and testing of continuous and discontinuous catalytic metal films as detection elements on a gallium nitride metal-semiconductor-field-effect transistor (MESFET) gas sensor for measuring combustion gas products in high temperature gas streams. Gas adsorbs and reacts on the metal surface. The steady state composition of adsorbed species changes the metal work function. The significant innovation is a gallium nitride (large bandgap) semiconductor device which will advance this emerging technology to high temperature (ca. 600 C) applications. The multiple catalytic metals: platinum, palladium/silver and rhodium have different sensitivities and detection limits. These differences can in principle be used to distinguish the effects of up to three concentration variables. This is the first time rhodium will have been used in this kind of sensor. The outcome of this work will be a proof of the concept that quantifying high temperature gas compositions is possible with the multiple catalytic gate FET sensor technology. A work product will be: (1) the isotherms for gas (propane, methane, propylene, NO, NO2 and CO) adsorption on polycrystalline films of Pt, PdAg and Rh; (2) investigation of anticipated significant interactions of multiple gases on these metals; (3) the documentation of any solid-state reactions between the metals and the gallium nitride substrate by x-ray, TEM and other surface techniques; and, (4) the mechanical and electrical effects on the FET structure in various gases and at temperatures as high as 850 C. The goal of the research is a robust sensor structure and composition that can be used to monitor combustion gas including automobile exhaust for "breakthrough" of the catalytic converter and possibly engine control for better efficiency. The potential commercial applications of the research is a sensor for monitoring emission to meet anticipated regulatory requirements for ultra-low-emissions-vehicles for the future. Other applications include a variety of combustion gas environments and monitoring and real-time control of refinery and other industrial chemical processes.