IEEE Access (Jan 2021)
Abruptly-Switching MoS₂-Channel Atomic-Threshold-Switching Field-Effect Transistor With AgTi/HfO₂-Based Threshold Switching Device
Abstract
A two-dimension (2D) atomic-threshold-switching field-effect transistor (ATS-FET) was implemented, by connecting an AgTi/HfO2-based threshold-switching (TS) device in series to the drain electrode of the 2D baseline-FET with molybdenum disulfide (MoS2) channel material. We optimized/developed the Ag/HfO2-based TS device because its characteristic is associated to the way how to achieve high performance of the 2D ATS-FET. By reducing the effective device area of the Ag/HfO2-based TS device down to $4~\mu \text{m}^{2}$ , low threshold voltage ( $\text{V}_{\mathrm {T}} \sim 0.42$ V), low threshold current (IT, drain current at the threshold voltage, $\sim 3.79 \times 10^{-11}$ A), and low VT variation (~0.09 V) were achieved. This is because the randomly formed filaments and electric field are better controlled with the scaled effective area. Next, the titanium (Ti)-injection barrier layer was inserted between the top electrode and the switching layer, while maintaining the optimized area in the TS device. The inserted Ti-injection barrier layer prevents the migration of Ag ions into the switching layer, enabling the stable TS operation even under the compliance current of $100~\mu \text{A}$ . Additionally, it locally restricts the region where the filaments are created inside the switching layer, resulting in a 17% lower VT variation and stable IT to approximately $\sim 1.5 \times 10^{-11}$ A in 100 cycles. Due to the low off-state leakage current and low variation characteristic of the optimized AgTi/HfO2-based TS device, the 2D ATS-FET (vs. 2D baseline-FET) shows the reduction of off-state leakage current (by $\sim 10^{2}$ in sub-threshold region) and the stable switching characteristic. The proposed 2D ATS-FET shows stably steep switching characteristics, e.g., sub-threshold swing under forward bias (~19 mV/decade) and reverse bias (~26 mV/decade), because of its abruptly switching characteristics of the TS device.
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