Threshold Voltage and On-Off Current Ratio Tuning of MoS₂ Transistor

Immunity to short channel effects and abrupt switching along with transparency and flexibility have made 2D molybdenum disulfide (MoS₂) transistors compelling to be used in digital logic circuits. However, difficulties in obtaining a positive threshold voltage (V_th) have hindered its application as in digital logics, an incorrect V_th can lead to large short-circuit power consumption during switching. Similarly, a poor ON/OFF current ratio (I_on/I_off) can cause impractically low output swings and high leakage.<br/>In this work, we employed the strategy of tuning the work function of gate metal using an ALD-grown metal plus thinning the gate oxide dielectric in order to reduce the trap states to shift the threshold voltage towards the positive direction and increase I_on/I_off. To achieve that, two series of transistors were fabricated using a bottom gate structure on silicon substrates. The transistor's channel lengths varied from 1.5 um to 40 um and the channel widths varied from 10 um to 160 um. In one series of transistors, niobium was deposited as the gate metal and 35 nm aluminum oxide (Al₂O₃) was used as the gate dielectric. In the second series of transistors, titanium nitride (TiN) and 12 nm Al₂O₃ were used as the gate metal and the gate dielectric respectively. A CVD-grown multilayer MoS2 was wet transferred to the structure and used as the gate channel.<br/>The electrical characteristics of the transistors were measured in the ambient environment and under dark conditions. A virtual source (VS) model was used to fit a curve on the measured data. The extracted transistors parameters from the fitted curves for the two series of transistors were compared. It was observed that the device performance enhanced significantly when TiN and thinner Al₂O₃ were deposited as the gate metal and the gate dielectric. While as-fabricated devices with Nb and 35 nm Al₂O₃ showed a negative threshold voltage of ~ -3 V, the device threshold voltage shifted toward the negative direction over time and by keeping the sample in the ambient environment. The I_on/I_off of these devices did not exceed 10⁴.<br/>The transistors using TiN gate metal and 12 nm Al₂O₃ with the W/L ratio of 160/10 showed the ON-OFF current ratio of 10⁶ with the ON current of 11.4 uA at V_ds of 1 V and V_gs of 5 V and the threshold voltage of 1.1 V. The average threshold voltage of the transistors fabricated using TiN gate metal and thin Al₂O₃ dielectric layer for different W/L ratios was 1.3 V. We were able to adjust the threshold voltage to the positive values and tune the on-off current ratio across 10⁴ to 10⁶. This study shows a comparably simple strategy for the fabrication of MoS₂ transistor for low power consumption logic circuits for industrial applications.