  We discuss the origin of an additional dip other than the charge neutrality
point observed in transfer characteristics of graphene-based field-effect
transistors. The double-dip is proved to arise from charge transfer between
graphene and metal electrodes, while charge storage at the graphene/SiO2
interface enhances it. Considering different Fermi energy from the neutrality
point along the channel and partial charge pinning at the contacts, we propose
a model which explains all features in gate voltage loops.
