An Ultralow Subthreshold Swing of 28 mV dec^-1 in Negative-Capacitance Thin-Film Transistors with Ferroelectric Zirconium-Aluminum Oxide Gate Oxide

Subthreshold swing (SS) below the thermionic limit is demonstrated for ZnO thin-film transistors (TFTs) integrated with ferroelectric (FE) zirconium-aluminum oxide (ZAO) gate oxide. The transfer characteristics of FE-ZAO/ZnO TFTs exhibit a substantial memory window of 5.03 ± 0.06 V, a high I_ON/I_OFF ratio of 1.5 × 10⁸, and an ultralow SS of 27.8 ± 2.2 mV dec^-1 during the reverse sweep over 4 orders of magnitude of the drain current at a drain voltage (V_DS) of 100 mV. An unstable polarization state is observed during the forward sweep, indicating SS below 60 mV dec^-1 at an ultralow V_DS of 0.1 mV. The negative differential resistance behavior in the output curves further confirms the negative-capacitance effect in FE-ZAO/ZnO TFTs. Finally, a logic inverter is demonstrated with a gain of 37 for cost-effective energy-efficient multistage logic circuits.