A simple route towards the reduction of surface conductivity in gas sensor devices

Surface conductivity caused by adsorbed water ions and molecules affects the performance of capacitive field effect transistor (FET) based gas sensors. Surface conduction leads to a slow decay of any voltage difference built-up between the sensitive surface and the gate electrode of the FET and thus to a drift in the sensor signal. In humid environments this problem becomes even more significant, as more water is becoming adsorbed. This leads to a cross-sensitivity of the sensor signal towards moisture. In this work we report on the application of a simple room temperature violet photochemical process to yield chemically anchored thin fluoropolymer films on the surface of such FET devices. The modified areas of the sensor surface were strongly water repellent. This led to a decrease in surface conductivity by more than three orders of magnitude at high relative humidities. Making the surface of the sensor chips hydrophobic resulted in elimination of the surface discharge caused by the leakage current even at high relative humidities.