Impact of Contact Resistance on the $f_T$ and $f_{\\max}$ of Graphene Versus $\\text{MoS}_2$ Transistors

A key challenge in making 2-D materials viable for electronics is reducing the contact resistance $\\rho _C$ of the source and drain, which can otherwise severely curtail performance. We consider the impact of contact resistance on the performance of transistors made with single-layer graphene and $\\text{MoS}_2$, two of the most popular 2-D materials presently under consideration for radio-frequency (RF) applications. While our focus is on the impact of $\\rho _C$, we include the impact of all the device parasitics. We consider a device structure based on the 7-nm node of the ITRS and use the unity-current-gain and unity-power-gain frequencies ($f_T$ and $f_{\\max}$) found from quantum-mechanical simulations, ballistic for graphene and with scattering for $\\text{MoS}_2$, as indicators of RF performance. We quantify our results in terms of the values of $\\rho _C$ needed to reach specific values of $f_T$ and $f_{\\max}$. In terms of peak performance (over all bias conditions), we show that graphene retains a significant edge over $\\text{MoS}_2$, despite graphene's poor output conductance, with $\\text{MoS}_2$ only being able to bridge the gap if considerably better contact resistances can be realized. However, with the bias current restricted to a technologically relevant value, we show that graphene loses much of its advantage, primarily due to a reduction in its transconductance $g_m$, and we show that $\\text{MoS}_2$ can then meet or exceed the performance of graphene via the realization of contact resistances already achieved in multilayer structures. Our values of $f_{T}$ for short-channel devices (around the 7-nm ITRS node) are shown to be consistent with experimental data for present-day long-channel devices, supporting our approach and conclusions.