Field-effect transistor using a solid electrolyte as a new oxygen sensor

A field-effect transistor (FET) using a solid electrolyte is proposed in the present study as a new oxygen sensor. The sensor is fabricated by depositing a thin layer of yttria-stabilized zirconia (YSZ) on a gate insulator of an insulated gate field-effect transistor (IGFET). As an IGFET has an ability to transform impedance, the potential change produced at the interface between the YSZ layer and a platinum gate electrode can be detected stably, even if the impedance of the YSZ is very high. The response of the fabricated sensor showed good reproducibility at 20 °C. A linear relationship between output voltage and logarithmic partial pressure of oxygen was obtained in the range from 0.01 to 1 atm. Sensitivity of the sensor was found to depend on the thickness of the Pt-gate electrode and sputtering conditions of the YSZ layer. Although selectivity to hydrogen and carbon monoxide was not good at room temperature, it could be improved by increasing the operating temperature to 100 °C. The developed sensor has several advantages including small size, low output impedance, and solid-state construction. It is potentially applicable to medical uses, process control, and automobiles.