A hydrogen gas sensor based on platinum-titanium-oxygen (Pt-Ti-O) gate silicon-metal-insulator-semiconductor field-effect transistors (Si-MISFETs) was developed. The sensor has a unique gate structure composed of titanium and oxygen accumulated around platinum grains on top of a novel mixed layer of nanocrystalline TiO(x) and superheavily oxygen-doped amorphous titanium formed on SiO(2)/Si substrates. The FET hydrogen sensor shows high reliability and high sensing amplitude (Delta V(g)) defined by the magnitude of the threshold voltage shift. Delta V(g) is well fitted by a linear function of the logarithm of air-diluted hydrogen concentration C (ppm), i.e., Delta V(g)(V) = 0.355 log C(ppm)-0.610, between 100 ppm and 1%2 This high gradient coefficient of Delta V(g) for the wide sensing range demonstrates that the sensor is suitable for most hydrogen-safety-monitoring sensor systems. The Pt-Ti-O structures of the sensor are typically realized by annealing Pt (15 nm)/Ti (5 nm)-gate Si-metal-oxide-semiconductor structures in air at 400 degrees C for 2 h. The Pt-Ti-O gate MIS structures were analyzed by transmission electron microscope (TEM), x-ray diffraction, Auger electron spectroscopy, and TEM energy dispersive x-ray spectroscopy. From the viewpoint of practical sensing applications, hydrogen postannealing of the Pt-Ti-O gate Si-MISFETs is necessary to reduce the residual sensing amplitudes with long tailing profiles. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483942]