**5. Nanomaterial-based FETs**

FET-based gas sensors have been expanded to sensors containing nanomaterials. Nanomaterialbased FETs have large surface-to-volume ratios, which contribute to high sensitivity and fast response and recovery times [3]. Nanomaterials allow for high-packing densities because of their intrinsic small dimensions [5]. This section briefly reviews FET-based gas sensors using nanomaterials such as CNTs, NWs, graphene, and transition metal chalcogenides.

### **5.1. CNT-based FETs**

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

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

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

layer. This FGFET was used for sensing H<sup>2</sup>

**Figure 7.** (a) Schematic illustration and (b) equivalent circuit diagram of a reported FGFET. Reprinted with permission

(500 ppm).

gate structure by a spraying process.

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

American Institute of Physics.

isolated by the SiO<sup>2</sup>

from Ref. [34]. Copyright 2003 Elsevier.

The fabrication of CNT-based FETs was first reported in 1998 [36, 37]. A typical CNT-based FET consists of a CNT, source and drain electrodes, insulating layer, and a substrate as the back gate [38]. Both individual CNTs and random networks of CNTs can be used to prepare CNT-based FETs. Chemical-gating effects of an individual single-walled CNT-based FET caused by exposure to gaseous NH<sup>3</sup> or NO<sup>2</sup> were reported in 2000 [39]. To date, CNT-based FETs have been applied to sensing H<sup>2</sup> [40], CH<sup>4</sup> [40], CO [40], CO<sup>2</sup> [41], NO<sup>2</sup> [40], NH<sup>3</sup> [40], H<sup>2</sup> S [40], alcohols [42], and breath samples [43].

To improve the sensitivity and selectivity of CNT-based FETs, they have been modified with nanoscale catalytic materials such as Pd [40, 44], Pt [40, 44], Rh [40], Au [40, 44], and Ag [44]. Furthermore, modifications with polymers [41], peptides [44], olfactory receptor proteins [45], and DNA [46, 47] have been reported.

#### **5.2. NW-based FETs**

#### *5.2.1. Gas-sensitive FETs using Si NWs*

As a gas-sensitive FET using one-dimensional nanomaterials, an application of an Si NW-based FET for sensing NH<sup>3</sup> was reported in 2006 [48]. After that, an FET-based sensor consisting of a highly ordered Si NW array on a bendable plastic substrate was prepared and applied to sensing NO<sup>2</sup> at parts per billion levels [49]. Furthermore, Si NW-based sensors have been applied to sensing H<sup>2</sup> [50].

Despite the potential of Si NW-based FETs for gas sensing, the sensitivity of bare Si NW-based FETs toward nonpolar volatile analytes is limited [51]. To overcome this, the native SiO<sup>2</sup> layer on the surface of the Si NWs has been chemically modified with silane monolayers [51]. Silane monolayer-modified Si NW-based FETs have been used for sensing nonpolar VOCs [51] and exhaled breath samples [52]. Modification with nanoparticles [50] has also been used to improve the responses of Si nanomaterial-based FETs to target analytes. In addition, an Si nanoribbon-based FET functionalized with an organic compound that is reactive toward nerve agents at sub-ppm levels has been reported [53].

## *5.2.2. Gas-sensitive FETs using metal oxide NWs or compound semiconductor NWs*

Metal oxide NWs such as In<sup>2</sup> O3 [54, 55], SnO<sup>2</sup> [56–58], and α-Fe<sup>2</sup> O3 [59] have also been applied to FET-based gas sensors. For example, an In<sup>2</sup> O3 NW-based FET has been used for sensing NO<sup>2</sup> and NH<sup>3</sup> [54].

Surface modification of NWs with nanoparticles has been used to improve the sensitivity and selectivity of gas-sensitive metal oxide NW-based FETs. To date, Pd [56, 58], Pt [55], Ag [55], Au [55], ZnO [57], and NiO [57] nanoparticles have been used to improve the properties of metal oxide NW-based FETs for gas sensing. For example, Moskovis et al. reported modification of SnO<sup>2</sup> NW-based FETs with Pd nanoparticles, and the application of this device to sensing H<sup>2</sup> [58]. In this work, an unusual sensitivity to H<sup>2</sup> in the charge depletion region of the device was reported [58]. This device was used for sensing a H<sup>2</sup> concentration range from 10 to 2500 ppm [58].

Compound semiconductor NWs have also been applied in FET-based sensors [60, 61]. Gao and coworkers applied NWs of InAs, which is a III−V semiconductor, to fabrication of a gas-sensitive FET [60]. This FET-based sensor was responsive to several gases and alcoholic vapors [60].

#### **5.3. 2D nanomaterial-based FETs**

Because of their high surface-to-volume ratios in molecular-level interactions, two-dimensional nanomaterials are attractive for use in FET-based sensors [5, 62]. Applications of 2D nanomaterials such as graphene and transition metal chalcogenides to FET-type gas sensors have been studied.

Since the potential of graphene-based sensors for gas sensing was first reported [63], other studies have investigated gas sensing using graphene-based FETs [62, 64]. A reported graphene-based FET is shown in **Figure 8** [64]. **Figure 8a** shows an atomic force microscopy (AFM) image of the FET based on graphene. Schematic diagram of the back-gate-type FET is shown in **Figure 8b** [64]. In the structure, the FET consists of a graphene sample connected

**Figure 8.** (a) AFM image and (b) schematic illustration of a graphene-based FET. Reprinted with permission from Ref. [64]. Copyright 2009 American Chemical Society.

with source and drain electrodes of Au/Cr, an SiO<sup>2</sup> -insulating layer, and a p-Si substrate as the back gate. In this report, the sensor was used for sensing NH<sup>3</sup> vapors [64].

*5.2.2. Gas-sensitive FETs using metal oxide NWs or compound semiconductor NWs*

[54, 55], SnO<sup>2</sup>

[58]. In this work, an unusual sensitivity to H<sup>2</sup>

the device was reported [58]. This device was used for sensing a H<sup>2</sup>

[56–58], and α-Fe<sup>2</sup>

NW-based FETs with Pd nanoparticles, and the application of this device to

O3

Surface modification of NWs with nanoparticles has been used to improve the sensitivity and selectivity of gas-sensitive metal oxide NW-based FETs. To date, Pd [56, 58], Pt [55], Ag [55], Au [55], ZnO [57], and NiO [57] nanoparticles have been used to improve the properties of metal oxide NW-based FETs for gas sensing. For example, Moskovis et al. reported modifi-

Compound semiconductor NWs have also been applied in FET-based sensors [60, 61]. Gao and coworkers applied NWs of InAs, which is a III−V semiconductor, to fabrication of a gas-sensitive FET [60]. This FET-based sensor was responsive to several gases and alcoholic vapors [60].

Because of their high surface-to-volume ratios in molecular-level interactions, two-dimensional nanomaterials are attractive for use in FET-based sensors [5, 62]. Applications of 2D nanomaterials such as graphene and transition metal chalcogenides to FET-type gas sensors

Since the potential of graphene-based sensors for gas sensing was first reported [63], other studies have investigated gas sensing using graphene-based FETs [62, 64]. A reported graphene-based FET is shown in **Figure 8** [64]. **Figure 8a** shows an atomic force microscopy (AFM) image of the FET based on graphene. Schematic diagram of the back-gate-type FET is shown in **Figure 8b** [64]. In the structure, the FET consists of a graphene sample connected

**Figure 8.** (a) AFM image and (b) schematic illustration of a graphene-based FET. Reprinted with permission from Ref.

O3

NW-based FET has been used for sensing

[59] have also been applied

in the charge depletion region of

concentration range from

O3

to FET-based gas sensors. For example, an In<sup>2</sup>

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

Metal oxide NWs such as In<sup>2</sup>

[54].

**5.3. 2D nanomaterial-based FETs**

[64]. Copyright 2009 American Chemical Society.

and NH<sup>3</sup>

cation of SnO<sup>2</sup>

10 to 2500 ppm [58].

have been studied.

sensing H<sup>2</sup>

NO<sup>2</sup>

As 2D nanomaterials, the transition metal chalcogenides MoS<sup>2</sup> [65], MoTe<sup>2</sup> [66], and WS<sup>2</sup> [67] have also been applied in FET-based gas sensors. **Figure 9a** shows a schematic illustration of a back-gate FET based on MoS<sup>2</sup> [5]. **Figure 9b** shows an optical image of MoS<sup>2</sup> -based FETs. In this FET, MoS<sup>2</sup> sheets were grown on an SiO<sup>2</sup> /Si substrate, with Ti/Au as the source and drain electrodes. This sensor was responsive to 20 ppb NO<sup>2</sup> and 1 ppm NH<sup>3</sup> [5].

**Figure 9.** (a) Schematic illustration and (b) optical image of a back-gate MoS<sup>2</sup> nanowire-based FET. Reprinted with permission from Ref. [5]. Copyright 2014 American Chemical Society.
