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

Tungsten oxide (WO₃) thin films are prepared via sol-gel route by spinning (WCl₆ in ethanol, 0.2 M) on Pt interdigitated Si/Si₃N₄ 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 WO₃ films to H₂ gas (0-250 ppm) in dry air and operating temperatures ranging from 25 to 250°C. Electrical tests confirmed an n-type response of the films. Although improved sensitivity (S=R_A/R_SENS) 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.