Explaining Steep-Slope Switching in Carbon Nanotube Dirac-Source Field-Effect Transistors

Dirac-source field-effect transistors (DS-FETs) have been proposed as steep-slope transistors for low-power switching. The steep-slope switching of a DS-FET originates from the "low-pass" energy filtering effect of the graphene Dirac source, which requires the number of modes in the graphene source to be lower than the ones in the channel. However, despite the fact that this requirement is not satisfied in a DS-FET with a carbon nanotube (CNT) channel, steep-slope switching has been experimentally demonstrated in such devices. In this article, we propose a mechanism of switching in CNT DS-FETs that is consistent with the small number of modes in a CNT. We argue that the CNT acts as a transverse momentum selector, which effectively introduces a bandgap in the graphene source, and thus, the Klein tunneling in the graphene n-p junction becomes band-to-band tunneling (BTBT). This makes the CNT DS-FET essentially a new type of tunneling field-effect transistor (TFET), which we call momentum-selector TFET (MS-TFET). Moreover, we study the impact of misorientation between graphene and CNT on the performance of a CNT MS-TFET. Finally, we show that scattering may actually be beneficial in such a device and could potentially enhance the on-current of a CNT MS-TFET.