The high-frequency capabilities of p-i-n and n-i-n doped-contact carbon nanotube field-effect transistors (CNFETs) are compared via simulations using a self-consistent, energy-dependent effective-mass Schrodinger-Poisson solver. Band-to-band tunneling, which is a characteristic feature of p-i-n CNFETs, can also occur in n-i-n CNFETs, and it is shown here that it reduces the unity-current-gain frequency f(T) in the latter devices. Generally, however, fT is higher in n-i-n CNFETs. For both types of device, fT increases with the chiral index of zig-zag tubes, but for different reasons. (C) 2009 Elsevier Ltd. All rights reserved.