Study Of Low Voltage ITO-based Thin Film Transistors

Thin film transistors have attracted much attention due to their promisingapplication in many fields such as smart card, portable equipment, sensors and activematrix display drive array. In fact, one of the major challenges in the development ofthin film transistors has been the rather high voltages needed for their operation（>20V）, making these devices impractical for low-priced applications. The key tolow-voltage application resides in the reduction of the threshold voltage and theinverse subthreshold slope. Both parameters are basically controlled by the gateinsulator. Gate dielectrics play an important role in fabrication of low-voltagetransistors. Recently, solid electrolytes as gate dielectric materials own muchattention. In a solid electrolyte, when a negative voltage is applied to the gateelectrode cations are attracted to the negatively charged gate electrode while anionsmigrate towards the electrolyte/organic semiconductor interface. This polarizationresults in the formation of electric double layers along the two interfaces in contactwith the electrolyte. The thickness of such electric double layers is about1nmindicating a high capacitance. Thin film transistors with such an electric-double-layercapacitor exhibit a low operation voltage and a high output current.In this thesis, we focus on the fabrication of low-voltage thin film transistors andthe work mainly includes two parts:1) the optimization of mesoporous SiO₂;2)demonstration of new gate insulator materials.1. According to the SEM images, the microstructure of the mesoporous SiO₂isnanocolumn and the distance between the columns is⁵nm. Here, we dipped themesoporous SiO₂into the SiO₂nanoparticles dispersion for different hours. We foundout that the leakage current minished indicating the appearance of SiO₂nanoparticlesin the aperture between the nanocolumns. Besides, the capacitance and ionconductivity of the mesoporous SiO₂has improved as the SiO₂nanoparticle enteredthe aperture because the nanoparticles in a electrolyte is good for ionization. As thedipping time increased, the performances of the thin film transistors changed. Thebest dipping time is around60minutes. In a word, it is a simple and outstandingmethod to improve the performance of mesoporous materials with electric doublelayers.2. Chitosan is a renewable polysaccharide biopolymer, which displays excellent film-forming ability, good adhesion, low or no toxicity, and biocompatibility.Chitosan has been used in many fields and it is the first time to be applied in thin filmtransistors as the gate dielectric materials. The capacitance of the chitosan film is7.47μF/cm²at0.1Hz indicating a low operation voltage and a high output current. TheITO-based thin film transistors with chitosan gate dielectric exhibit excellentperformances, good stability and reproducibility. The subthreshold gate voltage swing,current on/off ratio, subthreshold gate voltage and field-effect mobility of such deviceare estimated to be110mV/dec,6×10⁵,-0.5V,3.65cm²/V· s, respectively.As-fabricated transistors are promising in bio-sensors applications.