  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.
