▎ 摘　　要

We simulate stationary current distribution in graphene ribbons in the presence of top gate potentials, by means of the nonequilibrium Keldysh-Green's function formalism within a tight-binding model. In the absence of magnetic fields and in the presence of a model potential barrier, we observe the Klein paradox, where electrons turn into holes in the gated region and again into electrons beyond it. We establish a connection between the band structure at the corner points of the Brillouin zone and Klein paradox, and give a pictorial description of conductive channels. In the presence of high magnetic fields, transport currents are chiral and flow along the edges of the ribbon. The intensity and sign of the potential barrier with respect to the Fermi energy influence the nature (electron/hole) of the carriers inside the gated region and determine the edge involved in the transport process. We demonstrate that manipulation of currents in the ribbon can be obtained by external gates. (c) 2007 Elsevier B.V. All rights reserved.