  We present a data-calibrated compact model of carbon nanotube (CNT)
field-effect transistors (CNFETs) including contact resistance, direct
source-to-drain and band-to-band tunneling currents. The model captures the
effects of dimensional scaling and performance degradations due to parasitic
effects and is used to study the trade-offs between the drive current and
leakage current of CNFETs according to the selection of CNT diameter, CNT
density, contact length, and gate length for a target contacted gate pitch. We
describe a co-optimization study of CNFET device parameters near the limits of
scaling with physical insight, and project the CNFET performance at the 5-nm
technology node with an estimated contacted gate pitch of 31 nm. Based on the
analysis including parasitic resistance, capacitance, and tunneling leakage
current, a CNT density of 180 CNTs/{\mu}m will enable CNFET technology to meet
the ITRS target of drive current (1.33 mA/{\mu}m), which is within reach of
modern experimental capabilities
