**3. Conclusions**

*2.3.4. TFT photoresponse in the UV range*

O and O<sup>2</sup>

150 Design, Simulation and Construction of Field Effect Transistors

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

~ 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>

**Figure 11.** Time dependence of the mobility and threshold voltage for a spray-coated ZnO TFT in different atmospheres:

levels below 10 ppm (glove-box).

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.

The wide bandgap of zinc oxide (*Eg*

, with H<sup>2</sup>

(a) in air; (b) in N<sup>2</sup>

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*, *I 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).
