*2.3.1. Solution-processed ZnO TFTs*

This means that, for *VDS* higher than *Vsat*, the channel current achieves a saturation regime, as

**Figure 6.** Characteristic curves of TFTs (a) output curves with characteristic linear and saturation regions; (b) transfer

*<sup>V</sup>*\_\_\_*DS*

where *μ* is the charge carrier mobility, *Cox* is the capacitance per unit area of gate dielectric layer

ing layer and *d*, its thickness) and *Vth* is the *threshold voltage*, a voltage associated to the presence of charged traps at the semiconductor/dielectric interface and the difference of the work function between the semiconductor and the dielectric material, which is necessary to achieve the flat-band condition in the transistor channel. The channel current in the saturation regime, on

Eq. (2) means that the square root of the channel current depends linearly on the gate voltage.

which intercepts the abscissa at *Vth* and which slope gives the transistor *transconductance, gm*:

∂ √ \_\_ *<sup>I</sup>* \_\_\_\_*<sup>D</sup>* <sup>∂</sup> *Vg* ]*VDS*=*cte*

<sup>2</sup>*<sup>L</sup>* ( ∂ √ \_\_ *<sup>I</sup>* \_\_\_\_*<sup>D</sup>* ∂ *Vg* ) 2

Even though the carrier mobility in a TFT can be obtained in different conditions, the saturation regime is very important on the transistor operation, the reason why most of the papers use the saturation mobility as one of the relevant parameters (along with *Vth*) used to evaluate

\_\_\_\_\_\_\_\_ *w Cox*

Combining with Eq. (2), the carrier mobility in the saturation regime can be calculated:

the vacuum electric permittivity, *k*, the dielectric constant of the insulat-

<sup>2</sup> ) *VDS for VDS* ≪ (*Vg* − *Vth*) (1)

<sup>2</sup> *for VDS* ≫ (*Vg* − *Vth*) (2)

(3)

(4)

(curve in blue in **Figure 6b**) may give a straight line

The drain-source current in the linear regime can be approximated by [41, 42]:

curve and I1/2 versus vg (linear scale) extrapolation of Vth and slope for saturation mobility determination;.

*<sup>L</sup>* (*Vg* − *Vth*)

*<sup>L</sup>* (*Vg* − *Vth* −

observed in the output curves of **Figure 6a**.

144 Design, Simulation and Construction of Field Effect Transistors

*/d*, with *ε<sup>0</sup>*

the other hand, can be given by [41, 42]:

Consequently, a plot of *(IDS)1/2* versus *Vg*

*<sup>g</sup><sup>m</sup>* <sup>=</sup> [

*μsat* =

the transistor performance.

*DS*, *lin* <sup>=</sup> *<sup>w</sup> <sup>C</sup>* \_\_\_\_\_\_*ox*

*DS*, *sat* <sup>=</sup> *<sup>w</sup> <sup>C</sup>* \_\_\_\_\_\_*ox*

*I*

*I*

(*Cox = C/A = kε<sup>0</sup>*

In the present section, we compare the results from the electrical characterization of ZnO TFTs with the active deposited by spray-pyrolysis and by spin-coating, annealed at different temperatures (300 and 500°C) after deposition. **Figure 7a–d** shows the output curves of the produced devices. Substrates are p-type (Boron) doped Si wafers with a thermally grown SiO<sup>2</sup> insulating layer (100 nm thick) used in a bottom-gate structure. Aluminum top drain and source electrodes were deposited on top of the ZnO layer, with a channel width of 5 mm and a channel length of 100 μm (*w/L* ratio of 50).

The output curves show that spray-coated devices present better transistor characteristics compared to spin-coated transistors. Operating currents of spray-coated devices are considerably higher (more than 7 times for devices annealed at 300 oC and more than 13 times for devices annealed at 500 oC) than the operating currents observed for devices produced by spin-coating. Moreover, spin-coated devices have much higher off currents (for *Vg* = 0 V), probably due to higher lateral leakage current (spin-coated films cover the whole substrate area whereas spray-coated films can be deposited selectively in a smaller active area) and/or higher leakage current through the gate dielectric (due to Leidenfrost effect, spray-pyrolysis deposition promote less cracks in the insulating SiO<sup>2</sup> bottom layer than spin-coating).

The transfer curves (**Figure 7e** and **f**) also show strong dependence on deposition method and annealing temperature. Spray-coated devices, as expected from the output curves, present much higher on/off ratios (about 10<sup>6</sup> against 10<sup>3</sup> ) and also saturation mobility values almost 10 times higher than spin-coated TFTs. Subthreshold swing (SS) values of spray-coated TFTs are smaller and do not present significant variation on annealing temperature as observed for spin-coating. Improved device characteristics of spray-coated devices can be explained by better film quality and crystallinity, which promotes less charge trapping and scattering. The temperature influence on the transfer curves for devices produced by the same deposition method indicates that at higher temperatures the semiconductor intrinsic conductivity increases due to more efficient elimination of organic residues and better film crystallinity.

**Figure 7.** Output curves for solution-processed ZnO TFTs prepared by spin-coating and spray-pyrolysis and annealed at temperatures. Transfer curves for a spin-coated and spray-coated TFTs annealed at different temperatures.


**Figure 8** depicts the characteristic TFT curves of spin-coated AZO devices produced using Al:Zn ratios which varied from 0% (pure ZnO) to 5%. One observes that a small addition of Al (1%) is responsible for a substantial increase (more than 10 times) on the operating current when compared to a pure ZnO TFT, corroborating the expectation of semiconductor doping. By increasing the Al concentration, however, the device current gradually decreases (for 5%, the currents are smaller than for pure ZnO). At Al concentrations equal and higher than 10%, the currents became so small that transistor characteristics could not even be observed. Such behavior can be explained that, for higher Al concentrations, the formation of insulating aluminum-related compounds (like aluminum oxide) becomes more efficient than the semiconductor doping, and the phase separation between semiconducting and insulating regions leads to poorer morphological properties of the film, reducing the overall conductivity.

**Figure 8.** Output (a–d) and transfer (e) curves from spin-coated AZO TFTs produced using different active layer

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides

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147

The transfer curves of **Figure 8e** show that the TFT with 1% AZO active layer has a higher on current (and, consequently, higher mobility in saturation) and a higher intrinsic conductivity as well, which affects negatively the threshold voltage and the on/off ratio. At this point, it is important to notice that to obtain good TFT performance, it is not important to only increase the material conductivity, but also improve the film quality and control the number of defects

The addition of a precursor like indium acetate hydrate (**Figure 3c**) to a zinc precursor solution does not cause doping like in AZO (differently than Al, In atoms do not substitute Zn atoms in the lattice), but the formation of a ternary compound, indium zinc oxide (IZO). In ternary compounds, the relative concentration metal atoms must be much higher compared to doped compounds to result on significant changes in the electrical properties of the material. **Figure 9a** and **b** show the

at the semiconductor/dielectric interface.

compositions.

**Table 1.** Electrical parameters obtained from the TFT data presented on **Figure 7**.

Higher intrinsic conductivity decreases both the threshold and the onset voltages, since more negative voltages are needed to deplete the transistor channel from intrinsic n-type charge carriers. **Figure 7e** also shows that higher annealing temperatures promote the reduction of electron traps at the semiconductor/dielectric interface, which are responsible to displace *Vth* and *Von* to positive values. **Table 1** summarizes the electrical parameters obtained from the curves presented on **Figure 7**.

## *2.3.2. TFTs based on ZnO-related compounds*

The electrical properties of solution processed ZnO films can change dramatically by incorporating other metallic elements in the precursor solution, obtaining doped ZnO or ternary or quaternary metal oxide alloys. When a small amount of a precursor as aluminum acetate dibasic (**Figure 3b**) is added to a zinc acetate solution, aluminum-doped zinc oxide (AZO) films can be obtained after precursor pyrolysis. Aluminum atoms can substitute zinc atoms in the crystalline lattice, originating a donor level close to the conduction band, increasing the material conductivity by doping.

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides http://dx.doi.org/10.5772/intechopen.78221 147

**Figure 8.** Output (a–d) and transfer (e) curves from spin-coated AZO TFTs produced using different active layer compositions.

**Figure 8** depicts the characteristic TFT curves of spin-coated AZO devices produced using Al:Zn ratios which varied from 0% (pure ZnO) to 5%. One observes that a small addition of Al (1%) is responsible for a substantial increase (more than 10 times) on the operating current when compared to a pure ZnO TFT, corroborating the expectation of semiconductor doping. By increasing the Al concentration, however, the device current gradually decreases (for 5%, the currents are smaller than for pure ZnO). At Al concentrations equal and higher than 10%, the currents became so small that transistor characteristics could not even be observed. Such behavior can be explained that, for higher Al concentrations, the formation of insulating aluminum-related compounds (like aluminum oxide) becomes more efficient than the semiconductor doping, and the phase separation between semiconducting and insulating regions leads to poorer morphological properties of the film, reducing the overall conductivity.

Higher intrinsic conductivity decreases both the threshold and the onset voltages, since more negative voltages are needed to deplete the transistor channel from intrinsic n-type charge carriers. **Figure 7e** also shows that higher annealing temperatures promote the reduction of electron traps at the semiconductor/dielectric interface, which are responsible to displace *Vth* and *Von* to positive values. **Table 1** summarizes the electrical parameters obtained from the

**Figure 7.** Output curves for solution-processed ZnO TFTs prepared by spin-coating and spray-pyrolysis and annealed at

temperatures. Transfer curves for a spin-coated and spray-coated TFTs annealed at different temperatures.

The electrical properties of solution processed ZnO films can change dramatically by incorporating other metallic elements in the precursor solution, obtaining doped ZnO or ternary or quaternary metal oxide alloys. When a small amount of a precursor as aluminum acetate dibasic (**Figure 3b**) is added to a zinc acetate solution, aluminum-doped zinc oxide (AZO) films can be obtained after precursor pyrolysis. Aluminum atoms can substitute zinc atoms in the crystalline lattice, originating a donor level close to the conduction band, increasing the material conductivity by doping.

curves presented on **Figure 7**.

*2.3.2. TFTs based on ZnO-related compounds*

146 Design, Simulation and Construction of Field Effect Transistors

**Table 1.** Electrical parameters obtained from the TFT data presented on **Figure 7**.

The transfer curves of **Figure 8e** show that the TFT with 1% AZO active layer has a higher on current (and, consequently, higher mobility in saturation) and a higher intrinsic conductivity as well, which affects negatively the threshold voltage and the on/off ratio. At this point, it is important to notice that to obtain good TFT performance, it is not important to only increase the material conductivity, but also improve the film quality and control the number of defects at the semiconductor/dielectric interface.

The addition of a precursor like indium acetate hydrate (**Figure 3c**) to a zinc precursor solution does not cause doping like in AZO (differently than Al, In atoms do not substitute Zn atoms in the lattice), but the formation of a ternary compound, indium zinc oxide (IZO). In ternary compounds, the relative concentration metal atoms must be much higher compared to doped compounds to result on significant changes in the electrical properties of the material. **Figure 9a** and **b** show the

**Figure 9.** Output (a) and transfer (b) curves for a spin-coated IZO (1:1) TFT; (c) transfer curves for spin-coated TFTs at different IZO/ZnO compositions.

characteristic curves of a spin-coated IZO TFT using a 1:1 In:Zn molar ratio and treated at 350°C. It presents much improved TFT properties when compared to a ZnO TFT fabricated using similar processing method (**Figure 7a**). A saturation mobility of 5.32 cm<sup>2</sup> V−1 s−1 and an on/off ratio of 4.3 × 10<sup>4</sup> is obtained, against 0.14 cm<sup>2</sup> V−1 s−1 and 1.3 × 10<sup>3</sup> for the pure ZnO TFT (first row of **Table 1**). Spray-coated TFTs comprising IZO layer were not produced due to the need of optimization of the spray deposition method for the solvent (2-methoxyethanol). However, the expectation is to obtain transistors with much superior performance by using spray-pyrolysis IZO active layer.

The transfer curves of TFTs comprising different active layers (pure ZnO, pure indium oxide and IZO at 1:2, 1:1 and 2:1 In:Zn molar ratios) are presented in **Figure 9c**. A gradual increase on the on current (and on the saturation mobility) is observed when the indium concentration increases from pure ZnO up to 1:1 In:Zn. For higher In concentrations, the increase in the on current is not significant. However, the off current (and the intrinsic conductivity as well) always increase with the indium concentration. As a result, the transistor performance improves due to the increase of the mobility until a In:Zn molar ratio of 1:1, but deteriorates for higher indium concentrations due to decrease of the on/off ratio and displacement of *Vth* and *Von* toward negative values. Once again, we observe that a too conductive material like pure indium oxide does not result on superior performance transistor. As observed in **Figure 9c**, the pure indium oxide transistor has a too low on/off ratio and is not suitable for switching applications.

measurements could not be observed for short time intervals like few minutes, a significant change can be observed after 5 h in air. Both the intrinsic and on current continuously decrease as a function of time until about 40 h in air. After 40 h, the transfer curve seems to stabilize, with no substantial change until 94 h. **Figure 10b** shows the transfer curves for another transistor which was fabricated and annealed according the same conditions, but it was left inside a glove-box, with oxygen and water content lower than 10 ppm. The transfer curves show

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides

http://dx.doi.org/10.5772/intechopen.78221

**Figure 11** presents the evolution in time of the saturation mobility and the threshold voltage

tion in both parameters was higher for the transistor exposed to air. Moreover, the saturation

negative whereas, in air, it is positive. Negative *Vth* values in n-type TFTs are more associated to the increase of the intrinsic conductive than to the presence of charged defects. On the other hand, positive values of *Vth* in a n-type semiconductor is better explained by charge traps at

The significant variation on the electrical behavior of the ZnO TFT by exposure to oxygen rich atmosphere cannot be used, however, as evidence that the electron conductivity in ZnO is due to oxygen vacancies. Adsorbed atmospheric oxygen species can actually act as electron traps in ZnO, decreasing the electrical conductivity, but this effect can occur independently

atmosphere.

, with H<sup>2</sup>

O and O<sup>2</sup>

149

, presents a small increase in the first 24 h.

). The varia-

, it is still

a slight variation in the first few hours, but also stabilize after about 40 h in N<sup>2</sup>

**Figure 10.** Transfer curves for a spray-coated ZnO TFT in different atmospheres: (a) in air; (b) in N<sup>2</sup>

for the transistor exposed to air and for the transistor left in inert atmosphere (N<sup>2</sup>

The threshold voltage, in both cases, shifts toward positive values. However, in N<sup>2</sup>

mobility decreases when exposed to air and, in N<sup>2</sup>

on which native defect generate free n-type charge carriers.

the semiconductor/dielectric interface.

levels below 10 ppm (glove-box).

#### *2.3.3. Atmosphere influence on TFT performance*

In Section 2.1 we discussed briefly about the nature of the native defects which are responsible to the n-type character of ZnO. Theoretical calculations demonstrate that oxygen vacancies cannot be responsible for the unintentional n-doping of ZnO [12, 34, 35, 39]. However, reports on the increase of conductivity by increasing the oxygen pressure during sputtering deposition [14, 28, 40] are frequently mentioned as evidence that they play a key role in metal oxides n-type conductivity.

Atmospheric oxygen can also influence the electrical properties of ZnO TFTs even after the device manufacture. In **Figure 10a**, the transfer curves of a spray-coated ZnO TFT deposited at 350°C are presented as a function of exposure time in air. Previously to the experiment, the samples were annealed, in vacuum, at 150°C, for 1 h. Although differences in the electrical

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides http://dx.doi.org/10.5772/intechopen.78221 149

characteristic curves of a spin-coated IZO TFT using a 1:1 In:Zn molar ratio and treated at 350°C. It presents much improved TFT properties when compared to a ZnO TFT fabricated using similar

**Figure 9.** Output (a) and transfer (b) curves for a spin-coated IZO (1:1) TFT; (c) transfer curves for spin-coated TFTs at

Spray-coated TFTs comprising IZO layer were not produced due to the need of optimization of the spray deposition method for the solvent (2-methoxyethanol). However, the expectation is to obtain transistors with much superior performance by using spray-pyrolysis IZO active layer. The transfer curves of TFTs comprising different active layers (pure ZnO, pure indium oxide and IZO at 1:2, 1:1 and 2:1 In:Zn molar ratios) are presented in **Figure 9c**. A gradual increase on the on current (and on the saturation mobility) is observed when the indium concentration increases from pure ZnO up to 1:1 In:Zn. For higher In concentrations, the increase in the on current is not significant. However, the off current (and the intrinsic conductivity as well) always increase with the indium concentration. As a result, the transistor performance improves due to the increase of the mobility until a In:Zn molar ratio of 1:1, but deteriorates for higher indium concentrations due to decrease of the on/off ratio and displacement of *Vth* and *Von* toward negative values. Once again, we observe that a too conductive material like pure indium oxide does not result on superior performance transistor. As observed in **Figure 9c**, the pure indium oxide transistor has a too low on/off ratio and is not suitable for switching applications.

In Section 2.1 we discussed briefly about the nature of the native defects which are responsible to the n-type character of ZnO. Theoretical calculations demonstrate that oxygen vacancies cannot be responsible for the unintentional n-doping of ZnO [12, 34, 35, 39]. However, reports on the increase of conductivity by increasing the oxygen pressure during sputtering deposition [14, 28, 40] are frequently mentioned as evidence that they play a key role in metal oxides n-type conductivity.

Atmospheric oxygen can also influence the electrical properties of ZnO TFTs even after the device manufacture. In **Figure 10a**, the transfer curves of a spray-coated ZnO TFT deposited at 350°C are presented as a function of exposure time in air. Previously to the experiment, the samples were annealed, in vacuum, at 150°C, for 1 h. Although differences in the electrical

V−1 s−1 and 1.3 × 10<sup>3</sup>

V−1 s−1 and an on/off ratio of

for the pure ZnO TFT (first row of **Table 1**).

processing method (**Figure 7a**). A saturation mobility of 5.32 cm<sup>2</sup>

is obtained, against 0.14 cm<sup>2</sup>

148 Design, Simulation and Construction of Field Effect Transistors

different IZO/ZnO compositions.

*2.3.3. Atmosphere influence on TFT performance*

4.3 × 10<sup>4</sup>

**Figure 10.** Transfer curves for a spray-coated ZnO TFT in different atmospheres: (a) in air; (b) in N<sup>2</sup> , with H<sup>2</sup> O and O<sup>2</sup> levels below 10 ppm (glove-box).

measurements could not be observed for short time intervals like few minutes, a significant change can be observed after 5 h in air. Both the intrinsic and on current continuously decrease as a function of time until about 40 h in air. After 40 h, the transfer curve seems to stabilize, with no substantial change until 94 h. **Figure 10b** shows the transfer curves for another transistor which was fabricated and annealed according the same conditions, but it was left inside a glove-box, with oxygen and water content lower than 10 ppm. The transfer curves show a slight variation in the first few hours, but also stabilize after about 40 h in N<sup>2</sup> atmosphere.

**Figure 11** presents the evolution in time of the saturation mobility and the threshold voltage for the transistor exposed to air and for the transistor left in inert atmosphere (N<sup>2</sup> ). The variation in both parameters was higher for the transistor exposed to air. Moreover, the saturation mobility decreases when exposed to air and, in N<sup>2</sup> , presents a small increase in the first 24 h. The threshold voltage, in both cases, shifts toward positive values. However, in N<sup>2</sup> , it is still negative whereas, in air, it is positive. Negative *Vth* values in n-type TFTs are more associated to the increase of the intrinsic conductive than to the presence of charged defects. On the other hand, positive values of *Vth* in a n-type semiconductor is better explained by charge traps at the semiconductor/dielectric interface.

The significant variation on the electrical behavior of the ZnO TFT by exposure to oxygen rich atmosphere cannot be used, however, as evidence that the electron conductivity in ZnO is due to oxygen vacancies. Adsorbed atmospheric oxygen species can actually act as electron traps in ZnO, decreasing the electrical conductivity, but this effect can occur independently on which native defect generate free n-type charge carriers.

Such effects can be explained by the adsorption of atmospheric species, specially molecular oxygen, to the film surface. As described in the previous section, oxygen from the air adsorbs to the ZnO surface and acts as free electrons traps, decreasing the semiconductor conductiv-

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides

by the recombination of the trapped electrons with the photogenerated holes [38], leaving free electrons in the bulk and, consequently, increasing the conductivity. The slow readsorption of oxygen molecules from the air is, therefore, the main mechanism behind the persistent

We made a brief review on the basic properties of semiconducting metal oxides and presented the advantages of using solution-processed metal oxides films as the active layer of high-performance thin-film transistors for transparent, low-cost and large-area applications. The presented results from ZnO TFTs indicate that spray-pyrolysis deposition has an enormous potential to provide devices with improved performance when compared to other deposition methods like spin coated, suggesting that further improvement can be achieved by using doped or ternary ZnO-related compounds like AZO or IZO. The observed dependence of the electrical properties of ZnO TFTs on the environment oxygen content and on the UV-light exposure endorse them as excellent candidates for gases, volatile compounds or UV-radiation sensors. The fact that ZnO TFTs present sensing responses that can be quantified by multiple parameters *(μsat*, *Von*, *Vth*,

*on/Ioff*, *SS*, etc.) is a great advantage compared to the commonly proposed sensing units which usually present variations in a single parameter (like resistance, conductance or capacitance).

The authors acknowledge the financial support from CNPq (scholarships from Molecular Biophysics graduate program, IBILCE/UNESP), FAPESP (grant # 2013/24461-7) and to the contribution from State University of São Paulo—UNESP (PROPe/PROPG), which made possible the publication of this work. We also thank Nanotechnology National Laboratory for

, Giovani Gozzi<sup>2</sup>

1 Physics Department, IBILCE, São Paulo State University (UNESP), São José do Rio Preto,

2 Physics Department, IGCE, São Paulo State University (UNESP), Rio Claro, SP, Brazil

and Lucas Fugikawa Santos<sup>2</sup>

\*

Agriculture – LNNA (CNPq/SISNANO/MCTI) for the XRD measurements.

, Guilherme R. De Lima<sup>1</sup>

\*Address all correspondence to: lucas.fugikawa@unesp.br

molecule

151

http://dx.doi.org/10.5772/intechopen.78221

ity. The photogeneration of electron–hole pairs causes the release of the adsorbed O<sup>2</sup>

photoconductivity effect observed in ZnO TFTs.

**3. Conclusions**

**Acknowledgements**

**Author details**

João P. Braga<sup>1</sup>

SP, Brazil

*I*

**Figure 11.** Time dependence of the mobility and threshold voltage for a spray-coated ZnO TFT in different atmospheres: (a) in air; (b) in N<sup>2</sup> , with H<sup>2</sup> O and O<sup>2</sup> levels below 10 ppm (glove-box).

#### *2.3.4. TFT photoresponse in the UV range*

The wide bandgap of zinc oxide (*Eg* ~ 3.37 eV) makes it a suitable UV sensing material since its response is not affected by visible light, differently to, for example, Si-based photosensors [32, 43–45]. Another interesting feature of ZnO-based devices is the occurrence of persistent photoconductive, that is, the material conductivity remains higher than the dark conductive even several hours after UV-light exposure. **Figure 12** shows this effect on a spray-coated ZnO TFT deposited at 350°C.The transistor was irradiated for 2min by a UV LED (peak at 355nm, irradiance of 68μW/cm<sup>2</sup> ) and then several transfer curves were recorded in a 6 h interval. The experiment shows that the saturation mobility increases more than 3 times and the threshold voltage shifts almost 15 V toward negative voltages by UV irradiation, taking more than 6 h return to the original values.

**Figure 12.** Electrical characteristics of ZnO TFT after UV exposure in air.

Such effects can be explained by the adsorption of atmospheric species, specially molecular oxygen, to the film surface. As described in the previous section, oxygen from the air adsorbs to the ZnO surface and acts as free electrons traps, decreasing the semiconductor conductivity. The photogeneration of electron–hole pairs causes the release of the adsorbed O<sup>2</sup> molecule by the recombination of the trapped electrons with the photogenerated holes [38], leaving free electrons in the bulk and, consequently, increasing the conductivity. The slow readsorption of oxygen molecules from the air is, therefore, the main mechanism behind the persistent photoconductivity effect observed in ZnO TFTs.
