Impact of Contact Resistance on the F(t) and F(max) of Graphene Versus MoS2 Transistors

A key challenge in making 2-D materials viable for electronics is reducing the contact resistance ρC 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 MoS2 , two of the most popular 2-D materials presently under consideration for radiofrequency (RF) applications. While our focus is on the impact of ρC , 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 (fT and fmax ) found from quantum-mechanical simulations, ballistic for graphene and with scattering for MoS2 , as indicators of RF performance. We quantify our results in terms of the values of ρC needed to reach specific values of fT and fmax . In terms of peak performance (over all bias conditions), we show that graphene retains a significant edge over MoS2 , despite graphene’s poor output conductance, with MoS2 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 gm , and we show that MoS2 can then meet or exceed the performance of graphene via the realization of contact resistances already achieved in multilayer structures. Our values of fT 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.