Abstract - The effects of doping on the performance of coaxially gated carbon nanotube (CNT) field-effect transistors for both zero Schottky-barrier (SB) and doped carbon nanotube contacts are theoretically investigated. For ultrascaled CNTFETs in which the source/drain metal contacts lie 50 nm apart, there is no MOSFET-like contact CNTFET (C-CNTFET) with an acceptable on/off current ratio using a CNT of diameter >= 1.5 nm and a source/drain voltage >= 0.4 V. For CNTFETs with source/drain metal contacts either 50 nm or 100 nm apart, there is an optimal doping concentration of 1e-3 dopants per atom. The maximum on/off current ratios for the 50 nm CNT/5 nm gate and the 100 nm CNT/10 nm gate SB-CNTFETs are 5e4 and 6e5, respectively. Performance metrics of delay time, cutoff...