How Do Quantum Effects Influence the Capacitance and Carrier Density of Monolayer MoS2 Transistors?

When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance (CG) becomes smaller than the oxide capacitance (Cox) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS2 as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS2 capacitance is smaller than its quantum capacitance, limiting the single-gated CG of an n-type channel to between 63% and 78% of Cox, for gate overdrive voltages between 0.5 and 1 V. Despite these limitations, for dual-gated devices, the on-state CG of monolayer MoS2 is 50% greater than that of silicon at 0.5 nm EOT and more than three times that of InGaAs at 1 nm EOT, indicating that such 2D semiconductors are promising for improved gate control of nanoscale transistors at future technology nodes.