Steep-Switching Fully Depleted Silicon-on-Insulator (FDSOI) Phase-Transition Field-Effect Transistor With Optimized HfO₂/Al₂O₃-Multilayer-Based Threshold Switching Device

The OFF-state resistance (R _OFF ) and threshold voltage (V _T ) of a HfO ₂ -based threshold-switching (TS) device for phase-transition fully depleted siliconon-insulator (FDSOI) device on silicon substrate can be improved, e.g., ROFF is increased by implementing a multilayered HfO ₂ /Al ₂ O ₃ (rather than HfO ₂ layer only). The HfO ₂ /Al ₂ O ₃ layer with the atomic layer deposition (ALD) cycle ratio of 3:1 (i.e., Hf:Al = 3:1) shows approximately 3.47 × 10 ¹¹ Ω of average ROFF and approximately 2.12 V of average VT. Next, VT can be modulated by adjusting the work function of the bottom electrode. We demonstrate that a high work function of the bottom electrode results in stronger built-in electric-field and tunneling, decreasing VT and ROFF. The Hf ₃ Al1-based TS device with platinum electrode shows approximately 0.88 V of average VT and approximately 2.81 × 10 ⁹ Ω of average ROFF. The phase-transition FDSOI device used in this work is fabricated by connecting the optimized TS device in series to the drain electrode of a baseline FDSOI device, whose channel length is 130 nm. The phase-transition FDSOI device shows the minimum subthreshold slope of 9 mV/decade owing to the abrupt resistive switching of the TS device. Additionally, it exhibits an increase in the ON/OFF-state current ratio by three times because of high R _OFF .