  Electric transport of double gated bilayer graphene devices is studied as a
function of charge density and bandgap. A top gate electrode can be used to
control locally the Fermi level to create a pn junction between the
double-gated and single-gated region. These bilayer graphene pn diodes are
characterized by non-linear currents and directional current rectification, and
we show the rectified direction of the source-drain voltage can be controlled
by using gate voltages. A systematic study of the pn junction characteristics
allows to extract a gate-dependent bandgap value which ranges from 0 meV to 130
meV.
