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The carbon monoxide (CO) gas sensing mechanism of the Au/MO/n-LTPS MOS Schottky diodes on a glass substrate has been studied with various metal oxides (MO) including SnO2, ZnO, and TiO2 as the sensing element. Because of the deposited SnO2 has the highest band gap and the largest surface to volume ratio morphology, thus leading the Schottky diode with SnO2 to gain the highest relative sensitivity ratio of ~546% to 100 ppm CO ambient under condition of 200°C and -3 V bias. These results are better or comparable to the reported CO sensors. We propose a band gap model to interpret the related sensing mechanism in details.