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A novel hydrogen gas sensor based on platinum-titanium-oxygen gate silicon-metal-insulator-semiconductor field-effect transistors (FETs) 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 TiOx and super heavily oxygen-doped amorphous titanium formed on SiO2/Si substrates. The sensing amplitude, ΔVg(V), of the FET hydrogen sensor is well fitted by a linear function of the common logarithm of air-diluted hydrogen concentration C(ppm), i.e., ΔVg(V)=0.355log C(ppm)-0.610, between 100 ppm and 1%. The gradient of 0.355 V/decade at 115°C is about ten times higher than the expected 38.5 mV/decade from Nernst equation. The threshold Voltage (Vth) of FETs is reproducible, and shows excellent uniformity over the 5-inch wafers, 3σVth of 178 mV. Packaged sensors working at 115 °C typically show power consumption of 122.5 mW at room temperature environments.

The influence of radiation damages to developed hydrogen gas sensor chips from γ-rays (60Co) and/or X-rays (synchrotron radiation) is manageably avoided for sensor operations even at extremely high integral doses such as 1.8 and/or 18 MGy. Platinum–titanium–oxygen (Pt–Ti–O) gate silicon–metal–insulator–semiconductor field-effect transistor (Si-MISFET) hydrogen gas sensors can work stably as hydrogen sensors up to about 270 °C and also show environmental hardness as follows: When nitrogen-diluted 10-ppm hexamethyldisiloxane (HMDS) was exposed to the sensor FETs for 40 min at a working temperature of 115 °C, large sensing amplitude (ΔV g) changed little within repetition errors before and after HMDS exposures. The variations of ΔV g among relative humidity of 20 and 80% are very small within ±4.4% around 50% under 40 °C atmosphere. The Pt–Ti–O sensors have been found to show large ΔV g of 624.4 mV with σΔV g of 7.27 mV for nine times repeated measurements under nitrogen-diluted 1.0%-hydrogen gas, which are nearly the same values of 654.5 mV with σΔV g of 3.77 mV under air-diluted 1.0%-hydrogen gas.

A novel 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 TiOx and super heavily oxygen-doped amorphous titanium formed on SiO 2 /Si substrates. The FET hydrogen sensors show long life times more than ten years as intrinsic chip life time by accelerated temperature aging test, and high sensing amplitude (ΔV g ), which is well fitted by a linear function of the logarithm of air-diluted hydrogen concentration C(ppm), i.e., ΔV g (V)=0.355Log C(ppm)-0.610, between 100 ppm and 1%. The gradient of 0.355V/decade at 115 °C is about ten times higher than that of Pt gate Si-MOSFETs hydrogen sensors. The threshold Voltage (Vth) of FETs is reproducible, and shows excellent uniformity over the 5-inch wafers, 3σVth of 178 mV.

We have investigated Pt/Ti-gate Si-metal–oxide–semiconductor field-effect transistors (MOSFETs) from the viewpoints of hydrogen gas sensors and also mother devices of Pt–Ti–O-gate Si-metal–insulator–semiconductor field-effect transistor (MISFET) hydrogen gas sensors. The Pt/Ti-gate Si-MOSFETs with uniform coverage of Ti layers over the gate area do not respond to hydrogen gases in general. The observed large variations of threshold voltage V th and sensing amplitude ΔV g for the as-processed Pt (15 nm)/Ti (5 nm)-gate Si-MOSFETs originate from the initial unintentional oxygen invasion into the Ti layers at the beginning of phosphorus-doped silicon glass deposition. The air-annealed and succeeding hydrogen post-annealing procedure for Pt (15 nm)/Ti (5 nm)-gate Si-MOSFET hydrogen gas sensors recovers the uniformity of V th distributions close to that of as-processed Pt (90 nm)/Ti (5 nm)-gate Si-MOSFETs and leads to a large sensing amplitude ΔV g. We have found a critical V th, V thc, at which the response to hydrogen gas exposure begins. We have introduced a percolation model in which the ratio of Ti contact area to the effective Pt contact area over the gate oxides will be reduced to zero in accordance with the increasing dose of oxygen invasion, at a point which probably corresponds to V thc.

Although the threshold voltages (Vth) of the as-processed Pt(15 nm)/Ti(5 nm)-gate Si-MOSFETs under same channel ion dose conditions show a large variation such as 0.846 V among several wafers, the air-annealing and succeeding hydrogen post-annealing procedure for the FETs hydrogen gas sensors leads to excellent uniform Vth distributions and large sensing amplitude ΔVg. The oxygen invasion process through Pt grain boundaries to amorphous Ti layers at 400 °C air-annealing for two hours is not a simple dopant diffusion process but super-heavily oxygen-doped process partly to grow nano-crystalline TiOx. The oxygen-invaded Ti layers change to a kind of new materials; novel mixing layers of nano-crystalline TiOx and super-heavily oxygen-doped amorphous Ti formed on SiO2/Si substrates. The Ti mixing layers change from metals to semiconductors or insulators. As the Ti layers are so thin like 5 nm, the total amount of oxygen invaded into Ti layers will be saturated and stabilized. From the device operation point of view, it is crucial to control the Vth precisely that the Ti novel mixing layers are thin and fully depleted. This is supported by the fact that the Vth change before and after air-annealing procedures can be well explained by the difference of vacuum work function between Pt and Ti.

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 TiOx and superheavily oxygen-doped amorphous titanium formed on SiO2/Si substrates. The FET hydrogen sensor shows high reliability and high sensing amplitude (ΔVg) defined by the magnitude of the threshold voltage shift. ΔVg is well fitted by a linear function of the logarithm of air-diluted hydrogen concentration C (ppm), i.e., ΔVg(V)=0.355 log C(ppm)−0.610, between 100 ppm and 1%. This high gradient coefficient of ΔVg 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 °C for...

Air-annealing Pt (15 nm)/Ti (5 nm) gate Si-metal–oxide–semiconductor field-effect transistors (Si-MOSFETs) at 400 °C for 2 h revives and enhances the hydrogen sensing activity of nearly all the inert sensor characteristics of the Pt (15 nm)/Ti (5 nm) gate Si-MOSFETs. The sensing amplitudes are changed from 0.0–0.16 V to 0.82–0.96 V under the air-diluted 1000 ppm-hydrogen gas exposure before and after the air annealing. A transmission electron microscope (TEM) and Auger analysis clearly show that one of the causes is the novel mixing of nanocrystalline TiOx and super heavily oxygen-doped amorphous Ti in the Ti-layer formed on the gate oxide SiO2.