▎ 摘　　要

Unlike commonly observed ambipolar graphene field-effect transistors (GFETs) that show a V-shape transfer curve with hole and electron conduction region switching at the Dirac point, all our GFETs with graphene from chemical vapor deposition show p-type unipolar conduction under a wide range of back-gate voltage sweeping. After evaporating 3 nm-thick Al thin film on graphene surface, the unipolar characteristic was changed to ambipolar. The reason for this conversion might be that the as-prepared GFETs were p-type doped during the device fabrication process, while Al film has an n-type doping effect on graphene, thus restoring the intrinsic characteristics of GFETs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772493]