**2. Gas-sensitive FETs and field-effect devices combined with catalytic metal gates**

Catalytic-gate FETs are one of types of gas-sensitive FETs. In 1975, Lundström et al. first reported a Pd-gate FET sensitive to hydrogen [11, 12]. Pioneering research on catalytic-gate FETs opened up the field of FET-based gas sensors and other gas-sensitive field-effect devices such as capacitor-based [13–17] and Schottky diode-based sensors [18, 19]. Catalytic-gate filed-effect devices feature a nanoscale layer of catalytic metals, such as palladium and platinum, as a gate electrode on insulating layers in a metal-insulator-semiconductor (MIS) structure [20]. **Figure 1** shows reported schematic illustrations of this structure and the threshold voltage shift of a Pd-gate FET that is sensitive to hydrogen [21]. In initial reports of catalyticgate FETs, Pd as a catalytic-gate electrode was deposited onto the insulating layer of the MIS structure of the FET [11, 12, 21]. **Figure 2** shows changes observed in the threshold voltage [11] on hydrogen introduction to Pd-gate FETs. The gas-sensitive mechanisms of catalytic-gate FETs and catalytic-gate field-effect devices have been described in earlier reviews [20, 21].

Porous metal gates in catalytic-gate field-effect devices have allowed for important progress in NH<sup>3</sup> sensing [20, 22]. **Figure 3** shows reported TEM observations of 3- and 7-nm-thick Pt layers evaporated onto SiO<sup>2</sup> . These thin Pt layers consist of discontinuous metals [22]. The choice of catalytic materials, the structure of the catalytic layer, and the operating temperature affect the sensitivity and selectivity of catalytic-gate field-effect devices [14, 15, 20]. Furthermore, the type of insulating materials used in the MIS structure also influences the responsive properties of gas-sensitive field-effect devices [16].

For operation at high temperatures, silicon carbide (SiC)-based FETs have been investigated. SiC is a wide-bandgap semiconductor, and can be used as a substrate for the MIS structure instead of the conventional Si substrate [17]. SiC can be used at high temperatures and harsh environments because of its chemical inertness [23–25]. SiC-based FETs have been applied to the sensing of CO [23], NH<sup>3</sup> [23, 24], NO<sup>2</sup> [24], and SO<sup>2</sup> [25]. As with conventional catalyticgate FETs using an Si substrate, the catalytic-gate material used in SiC-based FETs influences the sensitivity and selectivity of the sensor [25].

Catalytic-gate devices consisting of high-electron mobility transistors (HEMTs) have also been studied for operation at high temperature. For example, GaN/AlGaN heterostructures that exhibit two-dimensional electron gas (2DEG) induced by spontaneous and piezoelectric polarization at the interface of the heterostructure have been applied to a catalytic-gate HEMT

For recognition of gaseous and volatile analytes from sensing results, two main methods have been used [3]. The conventional recognition method uses selective sensors with specific receptors designed for selective interaction with target analytes [3, 6]. Another recognition method uses a combination of cross-reactive sensor arrays and pattern recognition methods [3, 6–8, 10]. These cross-reactive sensor arrays consist of gas sensors that are responsive to a broad range of analytes and have differential sensitivities. To date, various gas sensors have been applied in sensor arrays [6, 8], including gas-sensitive FETs. In this chapter, research on the combination

of FET-based sensor arrays and pattern recognition methods is briefly reviewed.

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

**metal gates**

NH<sup>3</sup>

ers evaporated onto SiO<sup>2</sup>

the sensing of CO [23], NH<sup>3</sup>

ties of gas-sensitive field-effect devices [16].

the sensitivity and selectivity of the sensor [25].

**2. Gas-sensitive FETs and field-effect devices combined with catalytic** 

Catalytic-gate FETs are one of types of gas-sensitive FETs. In 1975, Lundström et al. first reported a Pd-gate FET sensitive to hydrogen [11, 12]. Pioneering research on catalytic-gate FETs opened up the field of FET-based gas sensors and other gas-sensitive field-effect devices such as capacitor-based [13–17] and Schottky diode-based sensors [18, 19]. Catalytic-gate filed-effect devices feature a nanoscale layer of catalytic metals, such as palladium and platinum, as a gate electrode on insulating layers in a metal-insulator-semiconductor (MIS) structure [20]. **Figure 1** shows reported schematic illustrations of this structure and the threshold voltage shift of a Pd-gate FET that is sensitive to hydrogen [21]. In initial reports of catalyticgate FETs, Pd as a catalytic-gate electrode was deposited onto the insulating layer of the MIS structure of the FET [11, 12, 21]. **Figure 2** shows changes observed in the threshold voltage [11] on hydrogen introduction to Pd-gate FETs. The gas-sensitive mechanisms of catalytic-gate FETs and catalytic-gate field-effect devices have been described in earlier reviews [20, 21].

Porous metal gates in catalytic-gate field-effect devices have allowed for important progress in

of catalytic materials, the structure of the catalytic layer, and the operating temperature affect the sensitivity and selectivity of catalytic-gate field-effect devices [14, 15, 20]. Furthermore, the type of insulating materials used in the MIS structure also influences the responsive proper-

For operation at high temperatures, silicon carbide (SiC)-based FETs have been investigated. SiC is a wide-bandgap semiconductor, and can be used as a substrate for the MIS structure instead of the conventional Si substrate [17]. SiC can be used at high temperatures and harsh environments because of its chemical inertness [23–25]. SiC-based FETs have been applied to

[24], and SO<sup>2</sup>

gate FETs using an Si substrate, the catalytic-gate material used in SiC-based FETs influences

Catalytic-gate devices consisting of high-electron mobility transistors (HEMTs) have also been studied for operation at high temperature. For example, GaN/AlGaN heterostructures that exhibit two-dimensional electron gas (2DEG) induced by spontaneous and piezoelectric polarization at the interface of the heterostructure have been applied to a catalytic-gate HEMT

[23, 24], NO<sup>2</sup>

sensing [20, 22]. **Figure 3** shows reported TEM observations of 3- and 7-nm-thick Pt lay-

. These thin Pt layers consist of discontinuous metals [22]. The choice

[25]. As with conventional catalytic-

**Figure 1.** Schematic illustrations of the (a) structure and (b) threshold voltage shift of a Pd-gate FET sensitive to hydrogen. Reprinted with permission from Ref. [21]. Copyright 1993 Elsevier.

**Figure 2.** Changes in the threshold voltage toward H<sup>2</sup> at different concentrations at 150°C. Reprinted with permission from Ref. [11]. Copyright 1975 American Institute of Physics.

**Figure 3.** Transmission electron micrographs of 3- and 7-nm thick porous Pt metal layers on SiO<sup>2</sup> . Reprinted with permission from Ref. [22]. Copyright 1987 Elsevier.

as a gas sensor [26]. In this report, the GaN/AlGaN-based HEMT combined with a Pt gate electrode was operated at about 400°C for sensing of H<sup>2</sup> , CO, C<sup>2</sup> H2 , and NO<sup>2</sup> .
