The Role of Nonidealities in the Scaling of MoS2 FETs

Abstract

2-D material FETs hold the promise of excellent gate control, but the impact of nonidealities on their performance remains poorly understood. This is because of the need, so far, to use computationally intensive nonequilibrium Green’s function (NEGF) simulations. Here, we therefore use a semiclassical model to investigate the role of nonidealities in the scaling of back-gated (BG) and top-gated (TG) monolayer MoS2 FETs. We verify the electrostatics and transport of the semiclassical model with density functional theory-based NEGF simulations and calibrate nonidealities, such as interface traps ( ${D}_{\textsf {it}}$ ) and Schottky contact barrier height ( $\phi _{\textsf {SB}}$ ) to experimental monolayer and bilayer MoS2 FETs. We find that among the nonidealities, ${D}_{\textsf {it}}$ has the strongest subthreshold swing impact with 70 mV/dec obtainable in BG devices for a ${D}_{\textsf {it}}$ of $5\times {10}^{11}$ cm−2eV $^{-1}$ , an equivalent oxide thickness (EOT) of 1 nm, and a channel length ( ${L}_{\textsf {ch}}$ ) of 5 nm. For scaled EOT, $\phi _{\textsf {SB}}$ only strongly impacts ${I}_{ \mathrm{\scriptscriptstyle ON}}$ for the TG case, as the overlapping gate thins the Schottky barriers in the BG case. We show in TG devices that a spacer of only 5 nm results in a 1000-fold drop in ${I}_{ \mathrm{\scriptscriptstyle ON}}$ because of the nonidealities. We propose positive spacer oxide charge as a solution and show that a charge density of above 1013 cm−2 is required to fully recover the device performance.