Electrolyte-gated transistor for CO2 gas detection at room temperature

The availability of a low-cost CO2 sensor with small package size and low power consumption is crucial to enter market for consumer application. This study is set out to deliver proof-of-concept and a hypothetical mechanism for a new CO2 sensing concept based on an electrolyte-gated transistor (EGT), which consists of the n-semiconducting metal oxide (MOX) In2O3 as channel-forming layer and ionic liquid (IL) [EMIM][BF4] as electrolyte. Gas measurements were performed under typical conditions for indoor and outdoor air quality evaluation, i.e. CO2 in the range 400-4000 ppm. The device behavior was reversible with response and recovery times (t90) of about 20 s in DC mode. Channel conductivity showed a linear CO2 sensitivity of 0.1 %/ppm and resolution <200 ppm at low CO2 concentrations (<1200 ppm). Fitting to the Clifford-Tuma model yielded a sensitivity coefficient of 175ppm−1 in the CO2 range 0-4000 ppm. The sensing mechanism is most likely based on simultaneous electrochemical reactions of H2O, O2, CO2. Basically, the CO2 sensitivity originates from the depletion of O2 in proximity to MOX, which leads to an amplified electrical current. The presented CO2 detection principle synergistically combines advantages of several device classes: Sensitivity of transistors, selectivity of electrochemical sensors, and simplicity of chemiresistors.