A novel Pt-Ti-O-gate Si-metal-insulator-semiconductor field-effect transistor (MISFET) hydrogen gas sensor has been proposed by Usagawa and Kikuchi (2010) [1]. The sensors consist of unique gate structures composed of Ti and oxygen accumulated regions around Pt grains on top of a novel mixing layer of nanocrystalline TiO(x) and superheavily oxygen-doped amorphous Ti formed on SiO(2)/Si substrates. The optimum Pt/Ti thickness and annealing conditions for most hydrogen safety monitoring sensor systems are obtained by annealing Pt(15 nm)/Ti(5 nm)-gate Si-MOS structures in air around 400 degrees C for 2 h. One of the advantages of the Pt-Ti-O-gate Si-MISFETs after 10 min of air-diluted 1000-ppm hydrogen exposure at 115 degrees C are reproducible and uniform threshold voltage of V(th) in addition to large sensing amplitudes at a practically important hydrogen concentration range between 100 ppm and 1%. The analysis of device characteristics of the Pt-Ti-O-gate Si-MISFETs hydrogen sensors concludes that the oxidation process of the Ti layer is consistently explained by an oxidation model that the oxygen invasion into Ti layer comes from open air through Pt grain boundaries and at the same time Ti will evacuate into the Pt surface through Pt grain boundaries. During the course of this process, the invading oxygen will be balanced with the evacuating Ti so that the Ti layer keeps nearly the same thickness with the as grown states. Ti and oxygen will remains around Pt grains named Ti and oxygen merged corridors. (C) 2011 Elsevier B.V. All rights reserved.