${\rm WO}_{3}$ Hydrogen Resistive Gas Sensor and Its Wide-Range Current-Mode Electronic Read-Out Circuit

Abstract

Tungsten oxide (WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) thin films are prepared via sol-gel route by spinning (WCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> in ethanol, 0.2 M) on Pt interdigitated Si/Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> substrates and annealed at 300°C for 12, 24, 96, and 200 hours. Films morphology and crystalline phase are characterized through scanning electron microscopy, atomic force microscopy, and glancing angle X-ray diffraction. The increasing of the annealing time shows a positive effect on the degree of crystallization but with no substantial influence on the crystallite size, surface area, and mean roughness of the films. Electrical tests are carried out using a current-mode dedicated read-out circuit to perform gas-sensing measurements of the polycrystalline WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films to H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas (0-250 ppm) in dry air and operating temperatures ranging from 25 to 250°C. Electrical tests confirmed an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -type response of the films. Although improved sensitivity ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SENS</sub> ) is achieved when decreasing the annealing time, best performances in terms of reproducibility and long-term stability of the response are obtained by annealing the film for 200 hours at 300°C temperature.