High-Mobility and Hysteresis-Free Flexible Oxide Thin-Film Transistors and Circuits by Using Bilayer Sol–Gel Gate Dielectrics

In this paper, we demonstrate high-performance and hysteresis-free solution-processed indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs) and high-frequency-operating seven-stage ring oscillators using a low-temperature photochemically activated Al 2 O 3 /ZrO 2 bilayer gate dielectric. It was found that the IGZO TFTs with single-layer gate dielectrics such as Al 2 O 3, ZrO 2, or sodium-doped Al 2 O 3 exhibited large hysteresis, low field-effect mobility, or unstable device operation owing to the interfacial/bulk trap states, insufficient band offset, or a substantial number of mobile ions present in the gate dielectric layer, respectively. To resolve these issues and to explain the underlying physical mechanisms, a series of electrical analyses for various single- and bilayer gate dielectrics was carried out. It is shown that compared to single-layer gate dielectrics, the Al 2 O 3 /ZrO 2 gate dielectric exhibited a high dielectric constant of 8.53, low leakage current density (∼10 –9 A cm –2 at 1 MV cm –1 ), and stable operation at high frequencies. Using the photochemically activated Al 2 O 3 /ZrO 2 gate dielectric, the seven-stage ring oscillators operating at an oscillation frequency of ∼334 kHz with a propagation delay of <216 ns per stage were successfully demonstrated on a polymeric substrate.