Fully integrated FET-type gas sensor with optimized signal-to-noise ratio for H₂S gas detection

The focus of gas sensor research has been on improving the response of individual sensors. However, in order to integrate sensors with CMOS circuits, a sensing system composed of an amplifier circuit with a voltage output is required. As such, in this work, we propose a novel gas sensing amplifier circuit composed of resistor- and FET-type gas sensors. Indium-gallium-zinc oxide (IGZO) is used as a sensing material for the detection of hydrogen sulfide (H₂S) gas. The H₂S gas sensing mechanism of the amplifier circuit is determined by the interplay between the resistor- and FET-type gas sensors. Also, the low-frequency noise characteristics of the resistor- and FET-type gas sensors are analyzed using carrier number fluctuation and Hooge's mobility fluctuation models, respectively. The signal-to-noise ratio of the amplifier circuit is accurately characterized as a function of the input voltage. The optimal operating bias condition is proposed, and the limit of detection of the amplifier circuit is obtained under this condition. The methodology demonstrated in this work can be applied to other types of amplifier circuits, contributing to the advancement of knowledge about integrated gas sensing systems.