**4. Suspended-gate FETs**

as a gas sensor [26]. In this report, the GaN/AlGaN-based HEMT combined with a Pt gate

**Figure 3.** Transmission electron micrographs of 3- and 7-nm thick porous Pt metal layers on SiO<sup>2</sup>

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>

trode. **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

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>

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

. Furthermore, a nanoscale layer of Pt was deposited on the YSZ film as a gate elec-

, CO, C<sup>2</sup>

H2

, and NO<sup>2</sup>

.

N4

. Reprinted with

N4 by

electrode was operated at about 400°C for sensing of H<sup>2</sup>

**3. Solid electrolyte-based FETs**

permission from Ref. [22]. Copyright 1987 Elsevier.

152 Different Types of Field-Effect Transistors - Theory and Applications

as the thickness of the Pt layer decreased.

coefficient for oxygen ions [28].

and SiO<sup>2</sup>

In 1983, Janata et al. reported an SGFET sensitive to dipolar molecules such as methanol and methylene chloride [29]. In the SGFET shown in **Figure 6**, fluid samples can penetrate into the gap between the insulating layer and the suspended metal mesh. Electrochemical surface modification using polypyrroles has been used to improve the selectivity of the SGFET [30]. This report described the preparation of SGFETs with differential selectivity by chemical modification with a polymer coating.

**Figure 6.** Schematic illustration of a suspended gate FET. Reprinted with permission from Ref. [29]. Copyright 1983 American Institute of Physics.

Improvement of fabrication processes is an important topic in SGFET research. Hybrid SGFETs prepared using an improved process and with diverse materials in the sensing layer have been reported [31]. In the fabrication process of hybrid SGFETs, the gate and body chip are prepared separately and then combined. This manufacturing technique has advantages over conventional methods because it allows for incorporation of a diverse range of sensitive materials in the structure. The flip-chip method has also been applied to the preparation of an SGFET for sensing ammonia [32]. In this report, a polyacrylic acid layer was formed on the gate structure by a spraying process.

The air gap in the gate structure of an SGFET causes undesirable effects on the sensing stability because of a lack of passivation, small W/L ratio, and a low gate capacity [33]. To overcome these drawbacks, research on SGFETs has been expanded to capacitively controlled FETs (CCFETs) [33] and floating-gate GasFETs (FGFETs) [34]. CCFETs contain an FET structure and a gas-sensitive capacitor with an air gap. FGFETs are a modification of CCFETs that use a floating gate for improved signal stability [34]. An FGFET with a hybrid-mounted gassensitive top electrode has been reported (**Figure 7a**) [34]. In this structure, the gas-sensitive capacitor and read-out transistor were integrated in one chip. **Figure 7b** shows the equivalent circuit diagram of the FGFET. The gate and the plate are electrically floating because they are isolated by the SiO<sup>2</sup> layer. This FGFET was used for sensing H<sup>2</sup> (500 ppm).

**Figure 7.** (a) Schematic illustration and (b) equivalent circuit diagram of a reported FGFET. Reprinted with permission from Ref. [34]. Copyright 2003 Elsevier.

Different FET-based sensors can be combined to extend the sensitivity range. For example, an SGFET responsive to high concentrations of H<sup>2</sup> and a catalytic-gate FET with good sensitivity for low concentrations of H<sup>2</sup> have been combined in one chip to increase the sensitivity range [35].
