Novel length scales in nanotube devices

We calculate the properties of p-n junctions, n-i junctions, and Schottky b arriers made on a single-wall carbon nanotube. In contrast to planar bulk j unctions, the depletion width for nanotubes varies exponentially with inver se doping. In addition, there is a very long-range (logarithmic) tail in th e charge distribution, extending over the entire tube. These effects can re nder traditional devices unworkable, while opening new possibilities for de vice design. Our general conclusions should apply to a broad class of nanot ube heterojunctions, and to other quasi-one-dimensional "molecular wire" de vices.