The performance characterization of the graphene/GaAs heterojunction energy harvester. (a) Raman spectrum of monolayer graphene. (b) Current versus voltage curve of graphene/GaAs with the RF source off. (c) Electronic band structure of independent graphene and GaAs. (d) Current characteristic versus time when switching the RF source off and on at a cycle of about 15 s on the graphene/GaAs wireless generator. (e) Voltage characteristic versus time when switching the RF source on and off on the graphene/GaAs wireless generator. (f) Schematic electronic band structure of the graphene/GaAs Schottky junction with the RF source on.