Impact of Contact Resistance on the fT and fmax of Graphene vs. MoS2 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.