Low-Voltage Transparent Indium–Zinc–Oxide Coplanar Homojunction TFTs Self-Assembled on Inorganic Proton Conductors

H₃PO₄-incorporated nanocolumnar SiO₂-based proton conductors show an enhanced electric double layer (EDL) capacitance value of ~8 μF/cm² at 20 Hz. The upper frequency limit of EDL formation is found to be higher than 10 kHz. Transparent indium-zinc-oxide (IZO) coplanar homojunction thin-film transistors gated by such proton conductors are fabricated by one metal shadow mask self-assembled method. The operating voltage is found to be as low as 0.6 V. The current ON/OFF ratio, the subthreshold swing, and field-effect mobility are estimated to be 6 × 10⁵, 68 mV/dec, and 12 cm²/V · s, respectively. Such transparent transistors hold promise for low cost and portable electrochromic devices and invisible biochemical sensors.