High-transconductance indium oxide transistors with a lanthanum-zirconium gate oxide characteristic of an electrolyte

We report solution-processed oxide thin-film transistors (TFTs) with an indium oxide (InO) channel and a lanthanum–zirconium oxide (LZO) gate insulator with a transconductance (of the mS order) that was two to three orders of magnitude higher than in common oxide TFTs. Analyses revealed that while the mobility was not high, the induced charge density in the channel was extremely high [typically >1.2 × 10¹⁴ cm^–2 per volt of V_G, corresponding to a high dielectric constant (ɛ_r) of >5000 for the InO/LZO structure]. In addition, the TFTs exhibited low operating voltages (1–2 V) and low subthreshold swing factors (SS, 70–90 mV decade⁻¹) that were close to the theoretical limit (∼60 mV decade⁻¹ at room temperature) of an ideal transistor. The transconductance decreased with decreasing humidity and was similar to that of a typical oxide TFT in a dry environment. Our data indicated that the high carrier density may arise from the formation of electric double layers in the presence of water molecules. Additionally, the crystallinity of the InO channel layer was dependent on the La/Zr ratio in LZO: the crystallinity significantly improved with an La/Zr ratio of 3/7 (high-transconductance TFTs) compared with a ratio of 7/3 (usual TFTs).