▎ 摘　　要

An ambipolar/unipolar conversion of conduction polarity in bottom-gate graphene field-effect transistor (FET) was realized by intended/unintended surface doping. Exposing the graphene FET in air made it fully p-type while covering graphene with Al nanofilm or poly(ethylene imine) (PEI) layer yielded a recovery of ambipolar conduction. The alteration of the conduction polarity in graphene FET was due to hole or electron-doping effect on graphene. Distinct changes in carrier mobility and current-voltage relationship were discussed between graphene with Al and PEI doping, and the dielectric screening by PEI was proposed as the possible mechanism. (C) 2013 AIP Publishing LLC.