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

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 MoS vertical channel between the two CNTs without field screening, achieving very strong gate modulation. As a result, our devices exhibit about 10 times higher on/off ratio (a maximum of 10), 10 times lower off current (10 A), and 560 times lower subthreshold swing (SS) (125 mV dec) compared to graphene/monolayer TMD/metal VFETs. In the comparison between multilayer MoS and monolayer MoS VFETs, rigid multilayer MoS forms a large air gap at the multilayer MoS/CNT/substrate, which reduces electric field transmission. In contrast, monolayer MoS 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 MoS/CNT VFET shows a 10 times higher on-current saturation and on/off ratio compared to the CNT/multilayer MoS/CNT VFET.