Observation of Negative Contact Resistances in Graphene Field-Effect   Transistors
  The gate-voltage (VG) dependence of the contact resistance (RC) in graphene field-effect transistors is characterized by the transmission line model. The RC-VG characteristics of Ag, Cu, and Au contacts display a dip around the charge neutrality point, and become even negative with Ag contacts. The dip structure is well reproduced by a model calculation that considers a metal-contact-induced potential variation near the metal contact edges. The apparently negative RC originates with the carrier doping from the metal contacts to the graphene channel and appears when the doping effect is more substantial than the actual contact resistance precisely at the contacts. The negative RC can appear at the metal contacts to Dirac-cone systems such as graphene. 
Ryo Nouchi, Tatsuya Saito, Katsumi Tanigaki
cond-mat.mtrl-sci cond-mat.mes-hall