Advanced semi-classical Monte Carlo modeling of Si, Ge, InGaAs, and MoS₂ n-channel FETs for novel CMOS

Scaling-down of silicon (Si) based complementary-metal-oxide-semiconductor (CMOS) technologies are approaching material limits. For high-performance applications, high thermal velocity channel materials, such as indium-gallium-arsenide (InGaAs) and germanium (Ge), are viable alternatives to Si to extend the limits of CMOS downscaling. The unique mechanical and electrical properties of two-dimensional atomic crystals, such as single-layer molybdenum disulfide (MoS₂), combined with soft, flexible, and curvilinear substrates, enable new device functionalities and concepts in the field of low-power flexible electronics not achievable with Si channels. While the intrinsic electron mobility of MoS₂ is rather low, strain engineering may provide a pathway for improving electron transport. Silicon, InGaAs, Ge, and MoS₂ n-channel MOSFETs were explored via first-principles computational tools including density functional theory and particle-based ensemble semi-classical Monte Carlo methods to better understand and enable the rational design of end-of-the-roadmap CMOS and potential beyond-CMOS technologies. The impact of contact geometry and transmissivity and gate length scaling on quasi-ballistic nanoscale Si, Ge, and InGaAs n-channel FinFETs was studied. FinFETs with end, saddle/slot, and raised source and drain contacts and the same saddle/slot contact geometry with different gate lengths, according to the projections of industry roadmaps, were simulated. Simulated Si FinFETs exhibited relatively limited degradation in performance due to non-ideal contact transmissivities, more limited sensitivity to contact geometry with non-ideal contact transmissivities, some contact-related advantage for Si 〈110〉 channel devices, and limited sensitivity to gate length scaling. Simulated InGaAs FinFETs were highly sensitive to modeled contact geometry, specific contact resistivity, the band structure model, and gate length scaling. Simulated Ge FinFETs showed substantial degradation due to non-ideal contact transmissivities, sensitivity to gate length scaling, and a large orientation-related advantage for Ge 〈110〉 channel devices. The impact of tensile strain on the intrinsic performance limits of monolayer MoS₂ n-channel MOSFETs was studied. 200 and 15 nm gate length MoS₂ MOSFETs with end contacts subject to different types and amounts of strain were simulated. Simulated MoS₂ MOSFETs displayed improved performance with strain due to lower effective mass and larger inter-valley separation, which is largely reduced due to non-ideal contact transmissivities.