▎ 摘　　要

We produced graphene-based field-effect transistors by contacting mono-and bi-layer graphene by sputtering Ni or Ti as metal electrodes. We performed electrical characterization of the devices by measuring their transfer and output characteristics. We clearly observed the presence of a double-dip feature in the conductance curve for Ni-contacted transistors, and we explain it in terms of charge transfer and graphene doping under the metal contacts. We also studied the contact resistance between the graphene and the metal electrodes with larger values of similar to 30 k Omega mu m(2) recorded for Ti contacts. Importantly, we prove that the contact resistance is modulated by the back-gate voltage.