**3. Solid electrolyte-based FETs**

Solid electrolytes can also be applied to FET-based sensors. For example, an FET-based oxygen sensor using yttria-stabilized zirconia (YSZ) as a solid electrolyte (**Figure 4**) has been reported [27]. In this sensor, a YSZ film was formed on an insulating layer consisting of Si<sup>3</sup> N4 and SiO<sup>2</sup> . Furthermore, a nanoscale layer of Pt was deposited on the YSZ film as a gate electrode. **Figure 5** shows responses of this sensor to oxygen and nitrogen (1 atm) [27]. At room temperature, a repeated stepwise response curve and a subsequent drift were observed. The response of the sensor showed a linear relationship against the partial pressure of oxygen in a logarithmic range between 0.01 and 1 atm. The sensitivity of the sensor to oxygen increased as the thickness of the Pt layer decreased.

To investigate the YSZ-based FET structure for use as an oxygen sensor, the crystalline structure and electrical properties were studied for a YSZ film deposited on a layer of Si<sup>3</sup> N4 by RF sputtering [28]. In the capacitance-voltage curve, hysteresis was observed, and was considered to be caused by the movement of oxygen ions and/or electrons in the YSZ film. This resulted in an unstable response at room temperature as mentioned above. Therefore, to increase the stability and quicken the response of the oxygen sensor at room temperature, the solid electrolyte-based FET would need to incorporate an electrolyte with a high diffusion coefficient for oxygen ions [28].

**Figure 4.** Schematic illustration of a YSZ-based FET. The YSZ-based FET is n-channel type and depletion-mode device. A nanoscale Pt layer is formed on a layer of YSZ. Reprinted with permission from Ref. [27]. Copyright 1988 American Institute of Physics.

**Figure 5.** Responses at 20°C of a YSZ-based FET against O<sup>2</sup> and N<sup>2</sup> . Reprinted with permission from Ref. [27]. Copyright 1988 American Institute of Physics.
