  Graphene/silicon (G/Si) heterojunction based devices have been demonstrated
as high responsivity photodetectors that are potentially compatible with
semiconductor technology. Such G/Si Schottky junction diodes are typically in
parallel with gated G/silicon dioxide (SiO$_2$)/Si areas, where the graphene is
contacted. Here, we utilize scanning photocurrent measurements to investigate
the spatial distribution and explain the physical origin of photocurrent
generation in these devices. We observe distinctly higher photocurrents
underneath the isolating region of graphene on SiO$_2$ adjacent to the Schottky
junction of G/Si. A certain threshold voltage (V$_T$) is required before this
can be observed, and its origins are similar to that of the threshold voltage
in metal oxide semiconductor field effect transistors. A physical model serves
to explain the large photocurrents underneath SiO$_2$ by the formation of an
inversion layer in Si. Our findings contribute to a basic understanding of
graphene / semiconductor hybrid devices which, in turn, can help in designing
efficient optoelectronic devices and systems based on such 2D/3D
heterojunctions.
