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This paper presents simulation of GaN high electron mobility transistor (HEMT) based device structures for the detection of toxic and hazardous gases like carbon monoxide (CO) and hydrogen (H2), respectively. AlGaN/GaN heterostructures show large potential as sensors due to the presence of 2-dimensional electron gas (2-DEG) at the heterointerface. Due to widebandgap material properties, GaN based devices are highly suitable for extreme-environment applications. The sensors are proposed selective towards specific targets by the two different gate structures. The simulated AlGaN/GaN based HEMT with Pt/AlGaN Schottky gate structure can detect hydrogen gas with the concentrations as low as ppb level and with the linear output variations from ~ ppb to 100 ppm level. A new gate structure based on nanocrystalline stannic oxide (α-SnO2) layer for the selective and sensitive detection of CO gas is proposed. We report that the AlGaN/GaN HEMT structure with Pt/α-SnO2/AlGaN Metal-Oxide-Semiconductor (MOS) gate can be used to detect sub-ppm level of CO with the linear response upto 500 ppm.