  Graphene/silicon heterostructures have attracted tremendous interest as a new
platform for diverse electronic and photonic devices such as barristors, solar
cells, optical modulators, and chemical sensors. The studies to date largely
focus on junctions between graphene and lightly-doped silicon, where a Schottky
barrier is believed to dominate the carrier transport process. Here we report a
systematic investigation of carrier transport across the heterojunctions formed
between graphene and highly-doped silicon. By varying the silicon doping level
and the measurement temperature, we show that the carrier transport across the
graphene/p++-Si heterojunction is dominated by tunneling effect through the
native oxide. We further demonstrate that the tunneling current can be
effectively modulated by the external gate electrical field, resulting in a
vertical tunneling transistor. Benefited from the large density of states of
highly doped silicon, our tunneling transistors can deliver a current density
over 20 A/cm2, about two orders of magnitude higher than previous
graphene/insulator/graphene tunneling transistor at the same on/off ratio.
