Transport Properties of Anatase--TiO2 Polycrystalline-Thin-Film Field-Effect Transistors with Electrolyte Gate Layers

We have fabricated anatase--TiO 2 polycrystalline-thin-film field-effect transistors (FETs) with poly(vinyl alcohol) (PVA), ion-liquid (IL), and ion-gel (IG) gate layers, and have tried to improve the response to gate voltage by varying the concentration of mobile ions in these electrolyte gate layers. The increase in the concentration of mobile ions by doping NaOH into the PVA gate layer or reducing the gelator in the IG gate layer markedly increases the drain--source current and reduces the driving gate voltage, which show that the mobile ions in the PVA, IL, and IG gate layers cause the formation of electric double layers (EDLs), which act as nanogap capacitors. In these TiO 2 -EDL-FETs, the slow formation of EDLs and the oxidation reaction at the interface between the surface of the TiO 2 film and the electrolytes cause unideal FET properties. In the optimized IL and IG TiO 2 -EDL-FETs, the driving gate voltage is less than 1 V and the ON/OFF ratios of the transfer characteristics are about $1\times 10^{4}$ at RT, and the nearly metallic state is realized at the interface purely by applying a gate voltage.