Effect of forming gas annealing on SnO2 sensing membranes in high-performance silicon-on-insulator extended-gate field-effect transistors

Abstract

In this study, we fabricated a dual-gate (DG) structured field-effect transistor (FET)-based pH sensor with an extended gate (EG) using a silicon-on-insulator (SOI) substrate. The DG-SOI-EGFETs achieved sensitivities significantly exceeding the theoretical maximum sensitivity (Nernstian limit) of conventional single-gate ion-sensitive FETs of ~59.16 mV/pH at 25 °C. For further improvement in the sensitivity and reliability of DG-SOI-EGFETs, we investigated the effects of forming gas annealing (FGA) temperature on the SnO2 film used as the EG sensing membrane. We evaluated the pH sensitivity of EG with SnO2 membranes after FGA at temperatures of 100–400 °C and investigated the hysteresis, drift effects, and response times. In addition, the chemical composition, crystallinity, sheet resistance, transmittance, and surface morphology of the SnO2 membranes with respect to FGA temperature were analyzed. As the temperature of FGA was increased, the SnO2 membrane showed increased crystallite size, transmittance, and surface roughness. However, the sheet resistance was decreased with increasing FGA temperature with a minimum at 300 °C before increasing again at 400 °C. Excellent sensor properties with high sensitivity, the fastest response time, and high reliability were obtained from FGA processing at 300 °C. Therefore, we found that the optimum FGA temperature for SnO2 films was 300 °C; this may facilitate the manufacture of EG sensing membranes for high-performance DG-SOI-EGFETs with improved sensitivity, stability, and reliability.