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-semi-conducting 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 (t(90)) 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, O-2, CO2. Basically, the CO2 sensitivity originates from the depletion of O-2 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.