  Negative differential conductance and tunneling characteristics of
two-terminal graphene devices are observed before and after electric breakdown,
respectively. The former is caused by the strong scattering under a high
E-field, and the latter is due to the appearance of a tunneling barrier in
graphene channel induced by a structural transformation from crystalline
graphene to disordered graphene because of the breakdown. Using Raman
spectroscopy and imaging, the presence of non-uniform disordered graphene is
confirmed. A memory switching effect of 100000% ON/OFF ratio is demonstrated in
the tunneling regime which can be employed in various applications.
