New NO2 sensor based on Au gate field effect devices

A new NO2 field effect gas sensor based on Au gates is demonstrated and the influence of gate morphology on sensor response is evaluated. A sensitization mechanism, for non-catalytic continuous gates, based on grain boundary diffusion is proposed. The sensors are fabricated as MOS (metal-oxide-semiconductor) capacitors with sputtered or thermal evaporated Au gates (at different substrate temperatures) with thickness between 75 and 960 nm. The devices’ sensitivity, in the range of 15–200 ppm of NO2 in dry air, depends strongly on gate morphology; shorter response times and larger voltage shifts are correlated with smaller grain sizes. Scanning-electron-microscope (SEM) images show that the microstructure is very stable after 5 months of gas exposure at temperatures up to 200°C. The sensors are selective to NO2 (with NO, H2 and CO as interfering gases) and selectivity depends also on gate structure.