  We demonstrate a tunneling and rectification behavior in bilayer graphene. A
stepped dielectric top gate creates a spatially modulated electric field, which
opens the band gap in the graphene and produces an insulating region at the p-n
interface. A current-voltage relationship exhibiting differential resistance
peak at forward bias stems from the tunneling current through the insulating
region at the p-n interface. The tunneling current reflects singularities in
the density of states modified by the electric field. This work suggests that
the effect of carrier charge tuning by external electric field in 2D
semiconductors is analogously to that by impurity doping in 3D semiconductors.
