  Using first-principle electronic structure calculations, we show a metal-
semiconductor transition of a metallic graphene nanoribbon with zigzag edges
induced by substitutional doping of Nitrogen or Boron atoms at the edges. A
field effect transistor consisting of a metal-semiconductor-metal junction can
then be constructed by selective doping of the ribbon edges. The
current-voltage characteristics of such a prototype device is determined by the
first-principle quantum transport calculations.
