CO2 gas sensing with an electrolyte-gated transistor using impedance spectroscopy

In this study, we present an electrolyte-gated transistor (EGT) as a sensor device for CO2 detection. Up to now, conclusive evidence could not be provided about the detailed sensing mechanism. The present report aims to deepen this knowledge. Electrochemical Impedance Spectroscopy (EIS) was found to be a suitable technique to analyze our system due to its capability to identify contributions from different relaxation processes to the total observed signal. Interpretation of EIS data was achieved by an a priori assumption of an Equivalent Circuit Model comprising all relevant relaxation processes expected in the sensor. It turns out that our EGT can be considered as a Mixed Ionic Electrical Conductor (MIEC), where either the gate potential or the AC frequency can control the dominance of either conduction path. Furthermore, EIS was evaluated as an additional operation mode for CO2 sensing with our EGT. Our results prove that EIS shows better sensing performance compared to direct current (DC) and FET (field effect transitor)-mode. Due to the analogy to electrostatic doping, our sensing mechanism can be termed a pseudo-gating process. Thus, the highest CO2 response was observed in the metal-oxide (MOX)-dominated frequency regime since the sensing mechanism basically exploits the O2-sensing capacity of the employed MOX. The sensor device was developed for applications in ambient CO2 sensing and offers a small-sized solution.