We produced graphene-based field-effect transistors by contacting mono- and bi-layer graphene by sputtering Cr, Ni, Ti or Nb as metal electrodes with a Si/SiO2 substrate used as the back-gate. 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 curves that we explain as the effect charge transfer from the contacts, causing different doping of graphene under the contacts and in the channel. We show that the double dip enhances the hysteresis in the transfer characteristics. Elucidation of the origin of the double dip is of technological importance since the observed distortion indicates a deterioration of the gate voltage response of the device. Although a possible drawback in circuitry applications, we also suggest that such a feature can be conveniently exploited to develop graphene based memory devices.