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

Tungsten oxide $({\rm WO}_{3})$ thin films are prepared via sol-gel route by spinning ( ${\rm WCl}_{6}$ in ethanol, 0.2 M) on Pt interdigitated ${\rm Si}/{\rm Si}_{3}{\rm N}_{4}$ substrates and annealed at 300 $^{\circ}{\rm C}$ 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 ${\rm WO}_{3}$ films to ${\rm H}_{2}$ gas (0–250 ppm) in dry air and operating temperatures ranging from 25 to 250$^{\circ}{\rm C}$. 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$^{\circ}{\rm C}$ temperature.