1,000,000 On/Off Ratio in Sub-1 nm Channel Length Carbon Nanotube/Monolayer MoS₂/Carbon Nanotube Vertical Transistors.

Abstract

Vertical field effect transistors (VFETs) using graphene and transition metal dichalcogenide (TMD) heterostructures are promising for downsizing the channel length to a monolayer TMD thickness of 0.65 nm. However, graphene/monolayer TMD/metal VFETs struggle with a low on/off ratio due to gate field screening by the graphene layer and a high off-state tunneling current caused by the large contact area. Here, we propose a 0.65 nm channel length VFET with a very high on/off current ratio made by cross-stacking top and bottom carbon nanotubes (CNTs) with a monolayer TMD in between. The ultranarrow junction area in the CNT/monolayer TMD/CNT VFET can significantly reduce the off-state tunneling current. Additionally, the gate field is transmitted from the sidewall of the bottom CNT to the monolayer MoS₂ vertical channel between the two CNTs without field screening, achieving very strong gate modulation. As a result, our devices exhibit about 10⁵ times higher on/off ratio (a maximum of 10⁶), 10⁵ times lower off current (10^-13 A), and 560 times lower subthreshold swing (SS) (125 mV dec^-1) compared to graphene/monolayer TMD/metal VFETs. In the comparison between multilayer MoS₂ and monolayer MoS₂ VFETs, rigid multilayer MoS₂ forms a large air gap at the multilayer MoS₂/CNT/substrate, which reduces electric field transmission. In contrast, monolayer MoS₂ bends significantly along the sidewall of the CNT, resulting in minimal air gap formation and enhancing the electric field effect in the channel. As a result, the CNT/monolayer MoS₂/CNT VFET shows a 10 times higher on-current saturation and on/off ratio compared to the CNT/multilayer MoS₂/CNT VFET.