**3. Background on FETs**

The ZnO field-effect transistor (FET) has been around for decades. The success of the device in meeting the technological demands has largely been dominated by the shrinking size of its physical geometry. It has an advantage as a junctionless (no p-n junctions) FET compared to conventional FETs [17, 21, 23–27, 53, 54]. There has been an introduction of new materials and heterojunction structures developed so as to move away from conventional silicon devices. High-K dielectrics have been introduced to replace the conventional SiO2 which should help maintain acceptable dielectric thicknesses while keeping gate leakage currents low [17, 21, 23–27, 53, 54].

Even with so many improvements being made to the device, the limits of FET scaling are approaching. The thickness of the oxide (tox) cannot be less than 1 nm due to high tunneling current and significant operational variation. The substrate doping is also very high which creates leakage and tunneling currents that are unacceptable to device operation.

#### **3.1 ZnO thin film transistors (TFTs)**

TFTs have also been fabricated using ZnO, mainly as thin film transistors for application in displays. **Figure 4** compares 20 ZnO TFTs fabricated by different authors [27, 53–71] using a variety of fabrication methods over the last 5 years. The graph is a plot of field effect mobility versus subthreshold slope which are two of the main parameters that describe the performance and efficiency of a device. The best device was fabricated by Bayraktaroglu et al. [70] with a SiO2 insulator and pulsed laser-deposited ZnO active channel layer. The device had a field effect mobility 110 cm<sup>2</sup> /Vs and an excellent subthreshold gate voltage swing of 109 mV/decade. This value of mobility is much higher than the value of around 1 cm<sup>2</sup> /Vs that is typically achieved with amorphous silicon TFTs in production displays. It is clear therefore that ZnO TFTs have considerable potential for application in high performance displays.

#### **Figure 4.**

*General literature review on TFTs looking at field effect mobility versus subthreshold slope of as-deposited and doped ZnO films.*

**11**

sensitivity.

**Figure 5.**

*and doped ZnO nanowires.*

<1.0 cm2

**3.3 Comparing ZnO NWFETs**

this research investigation.

**4. Biosensors**

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

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

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

high field effect mobility which is normally above 200 cm2

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

*Literature review on nanowire FETs looking at field effect mobility versus subthreshold slope of as-deposited* 

**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

have lower mobility values. Most of the top-down fabricated devices have mobility

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

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

/Vs with around three papers giving a mobility >10.0 cm2

/Vs; whereas the top-down

/Vs. The difference

#### **Figure 5.**

*Nanowires - Recent Progress*

is as outlined in [52].

**3. Background on FETs**

unacceptable to device operation.

field effect mobility 110 cm<sup>2</sup>

application in high performance displays.

1 cm<sup>2</sup>

**3.1 ZnO thin film transistors (TFTs)**

surface roughness <1.5 nm. Other tools such as RIE and ion beam etch produce roughness >5 nm. The fabrication process for the complete ZnO NWFET structure

The ZnO field-effect transistor (FET) has been around for decades. The success of the device in meeting the technological demands has largely been dominated by the shrinking size of its physical geometry. It has an advantage as a junctionless (no p-n junctions) FET compared to conventional FETs [17, 21, 23–27, 53, 54]. There has been an introduction of new materials and heterojunction structures developed so as to move away from conventional silicon devices. High-K dielectrics have been introduced to replace the conventional SiO2 which should help maintain acceptable dielectric thicknesses while keeping gate leakage currents low [17, 21, 23–27, 53, 54]. Even with so many improvements being made to the device, the limits of FET scaling are approaching. The thickness of the oxide (tox) cannot be less than 1 nm due to high tunneling current and significant operational variation. The substrate doping is also very high which creates leakage and tunneling currents that are

TFTs have also been fabricated using ZnO, mainly as thin film transistors for application in displays. **Figure 4** compares 20 ZnO TFTs fabricated by different authors [27, 53–71] using a variety of fabrication methods over the last 5 years. The graph is a plot of field effect mobility versus subthreshold slope which are two of the main parameters that describe the performance and efficiency of a device. The best device was fabricated by Bayraktaroglu et al. [70] with a SiO2 insulator and pulsed laser-deposited ZnO active channel layer. The device had a

of 109 mV/decade. This value of mobility is much higher than the value of around

*General literature review on TFTs looking at field effect mobility versus subthreshold slope of as-deposited and* 

displays. It is clear therefore that ZnO TFTs have considerable potential for

/Vs that is typically achieved with amorphous silicon TFTs in production

/Vs and an excellent subthreshold gate voltage swing

**10**

**Figure 4.**

*doped ZnO films.*

*Literature review on nanowire FETs looking at field effect mobility versus subthreshold slope of as-deposited and doped ZnO nanowires.*
