**3.2 Nanowire field-effect transistors (FETs)**

Emerging nonplanar devices [17, 21] are being researched to prolong the future progress for FETs. Devices based on quasi-one-dimensional (1-D) nanostructures are still at an embryonic stage from an industrial point of view. These nanostructures include the following: nanowires, nanobelts, nanoribbons, and nanoneedles [72, 73]. This review is interested in nanowire FETs which are also being researched for application in biosensors because the high surface-to-volume ratio provides high sensitivity.

#### **3.3 Comparing ZnO NWFETs**

**Figure 5** compares 15 different ZnO NWFETs fabricated by different authors using a variety of methods [22, 74–86]. The graph is plotted with field effect mobility against the subthreshold slope, which are two important device parameters that determine ZnO NWFET performance. The nanowires were fabricated using top-down and bottom-up (self-assembled) processes. Self-assembled processes tend to display very high field effect mobility which is normally above 200 cm2 /Vs; whereas the top-down have lower mobility values. Most of the top-down fabricated devices have mobility <1.0 cm2 /Vs with around three papers giving a mobility >10.0 cm2 /Vs. The difference in the mobility may be due to the fact that self-assembled nanowires are single-crystal, whereas top-down nanowires are polycrystalline. Nonetheless, top-down techniques are desirable as they currently pave way for mass production and will be pursued in this research investigation.

#### **4. Biosensors**

A biosensor is defined by the International Union of Pure and Applied Chemistry (IUPAC) as "a self-contained integrated device that is capable of providing specific quantitative or semiquantitative analytical information using a biological recognition element (biochemical receptor), which is retained in contact direct with a transduction element" [87]. A biosensor is a "more-than-Moore device" because it

incorporates functionalities that do not necessarily scale according to Moore's law. Under the roadmap, the device falls under the category of sensors and actuators. Other categories include analogue/RF, passives, HV power, and biochips [88, 89].

**Figure 6** shows a typical structure of a biosensor [90–92]. The biomolecules are contained within an analytic solution and attach themselves to immobilized enzymes or immune-agents on the linkers. Linkers in turn are attached to the transducer. The transducer then converts the charge on the analyte into an electrical signal which is then transmitted for data processing. Biosensors can be considered as part of the research field known as "chemical sensors" in that a biological mechanism is used for analyte detection within an analyte solution [93–95]. Quasi-one-dimensional nanostructures have a greater surface-to-volume ratio compared to planar structures and are therefore expected to be more sensitive than planar sensors [93–95].

Nanowires are the same as nanorods. The words can be used interchangeably [80]. These have received enormous attention due to their suitable properties for designing novel nanoscale biosensors. For example, the dimensions of ∼1–100 nm are similar to those of many biological entities, such as nucleic acids, proteins, viruses, and cells [79]. In addition, the high surface-to-volume ratios for nanomaterials allow a large proportion of atoms in the bio-analyte to be located at or close to the surface. Moreover, some nanowire materials have surfaces that can easily be chemically

#### **Figure 6.**

*Typical structure of a biosensor. The biomolecules are contained within an analytic solution and attach themselves to immobilized enzymes or immune-agents on the receptors. The transducer then converts the energy signal produced into an electrical signal which is then transmitted for data processing. [22].*

**13**

applications [123].

*ZnO Nanowire Field-Effect Transistor for Biosensing: A Review*

modified which makes them significant candidates for biosensors [79, 80]. There are a number of nanostructure-based electrical biosensors which include single-wall carbon nanotubes (SWCNT), nanowires, nanogaps, nanochannels, and nanoelectromechanical (NEM) devices. The project will focus on nanowire-based devices as they have considerable potential for electrical biosensing that offer the possibility

Over the past decade, silicon nanowires have been the most researched for application as biochemical sensors [97–108]. Silicon nanowires are of interest for a number of reasons, for example, the material is well known and is compatible with CMOS integrated circuits for the development of sensor systems [97–108]. The nanowire is expected to have high surface-to-volume ratios which give high sensitivity and the electrical sensing will give real-time label-free detection without the use of expensive optical components. Mass manufacturing is also a main advantage for silicon and is critically important for nanowire biosensor applications because of the widespread uptake of biosensors in "point-of-care" settings, the biosensor needs

A number of fabrication methods are well established for silicon nanowires which utilize both bottom-up and top-down methods (these methods are called hybrids). It still remains that bottom-up techniques have the advantage of simplicity [97–108]. Bottom-up methods are still limited due to the alignment problem. The hybrid methods require further nanowire technologies to achieve alignment, such as electric field or fluid-flow-assisted nanowire positioning to locate the nanowires between lithographically defined source and drain electrodes. The technique is interpreted as a hybrid between bottom-up and top-down. Top-down methods overcome these problems, and several researchers have used advanced lithography techniques to fabricate single-crystal silicon nanowires on silicon-on-insulator (SOI) substrates. SOI wafers are expensive and to overcome the problem some researchers [109] have devised alternatives to SOI. The electrical output characteristics of silicon nanowires are good and they are well suited for biosensing applications. The sensitivity range for most silicon-nanowire based biosensors is between

ZnO is investigated as it is expected to be more sensitive than Si due to its wider bandgap [109]. This is observed by comparing **Table 2** with **Table 3**. ZnO devices show results comparable to silicon devices; especially looking at response time and limit of detection. It is required that biosensors should have the liquid reference electrode. There are many different types of ZnO nanostructures being used for sensing application and **Table 2** compares the ZnO nanostructures such as nanotetrapods, nanocombs, and nanorods used for biosensing [110, 121]. Nanotetrapods [123] are like nanorods but with four single crystalline legs. Most of the ZnO devices were synthesized by vapor phase method and then transferred on Au electrode to form a multiterminal network for the sensor receptors. Like all other bottom-up ZnO nanostructures discussed here, they are transferred to a surface of a working electrode to form a thin layer to modify the transducer. The devices have low sensitivity but the nanotetrapods exhibit good detection limit down to ~1.0 nM. The researchers [123] did not explain why the nanostructures possess low sensitivity but its three-dimensional features have the potential for multiterminal communication

of portable assays in a variety of point-of-care environments [48, 90, 96].

*DOI: http://dx.doi.org/10.5772/intechopen.93707*

**4.1 Silicon biosensors**

to be disposable [97–108].

50 and 400 mV [97–134].

**4.2 Comparing ZnO nanowire biosensors**

modified which makes them significant candidates for biosensors [79, 80]. There are a number of nanostructure-based electrical biosensors which include single-wall carbon nanotubes (SWCNT), nanowires, nanogaps, nanochannels, and nanoelectromechanical (NEM) devices. The project will focus on nanowire-based devices as they have considerable potential for electrical biosensing that offer the possibility of portable assays in a variety of point-of-care environments [48, 90, 96].
