Let us first consider the case of the undoped ribbon. When the gate voltage coincides with the average between the contact voltages (i.e., VGS=VDS/2), the energy of the charge carriers falls in the transition energy range between the valence and the conduction bands, where the density of states is minimum, and therefore the current is minimum. Instead, when increasing (decreasing) the gate voltage, the graphene band structure shifts towards lower (higher) energies. As a consequence, the energy of the charge carriers falls in the range of the conduction (valence) bands, where the electron (hole) concentration is large, and thus the current ID increases. This gives rise to a typical ambipolar transport behavior. In particular, in the case of the undoped ribbon, the transfer characteristic is symmetric with respect to VGS=VDS/2, as an effect of the symmetry of the dispersion relations with respect to the midgap energy.