  We describe a robust technique for the fabrication of high performance
vertically scaled n-doped field-effect transistors from large band gap carbon
nanotubes. These devices have a tunable threshold voltage in the
technologically relevant range (-1.3V < V_th < 0.5V) and can carry up to 5-6
muA of current in the on-state. We achieve such performance by exposure to
potassium (K) vapor and device annealing in high vacuum. The treatment has a
two-fold effect to: (i) controllably shift V_th toward negative gate biases via
bulk doping of the nanotube (up to about 0.6e/nm), and (ii) increase the
on-current by 1-2 orders of magnitude. This current enhancement is achieved by
lowering external device resistance due to more intimate contact between K
metal and doped nanotube channel in addition to potential reduction of the
Schottky barrier height at the contact.
