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 ρ
_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 MoS
₂
, two of the most popular 2-D materials presently under consideration for radiofrequency (RF) applications. While our focus is on the impact of ρ
_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 MoS
₂
, as indicators of RF performance. We quantify our results in terms of the values of ρ
_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 MoS
₂
, despite graphene's poor output conductance, with 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 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.