Performance limits of transition metal dichalcogenide (MX₂) nanotube surround gate ballistic field effect transistors

We theoretically analyze the performance of transition metal dichalcogenide (MX₂) single wall nanotube (SWNT) surround gate MOSFET, in the 10 nm technology node. We consider semiconducting armchair (n, n) SWNT of MoS₂, MoSe₂, WS₂, and WSe₂ for our study. The material properties of the nanotubes are evaluated from the density functional theory, and the ballistic device characteristics are obtained by self-consistently solving the Poisson-Schrödinger equation under the non-equilibrium Green's function formalism. Simulated ON currents are in the range of 61–76 μA for 4.5 nm diameter MX₂ tubes, with peak transconductance ∼175–218 μS and ON/OFF ratio ∼0.6 × 10⁵–0.8 × 10⁵. The subthreshold slope is ∼62.22 mV/decade and a nominal drain induced barrier lowering of ∼12–15 mV/V is observed for the devices. The tungsten dichalcogenide nanotubes offer superior device output characteristics compared to the molybdenum dichalcogenide nanotubes, with WSe₂ showing the best performance. Studying SWNT diameters of 2.5–5 nm, it is found that increase in diameter provides smaller carrier effective mass and 4%–6% higher ON currents. Using mean free path calculation to project the quasi-ballistic currents, 62%–75% reduction from ballistic values in drain current in long channel lengths of 100, 200 nm is observed.