  Due to its high charge carrier mobility, broadband light absorption, and
ultrafast carrier dynamics, graphene is a promising material for the
development of high-performance photodetectors. Graphene-based photodetectors
have been demonstrated to date using monolayer graphene operating in
conjunction with either metals or semiconductors. Most graphene devices are
fabricated on doped Si substrates with SiO2 dielectric used for back gating.
Here, we demonstrate photodetection in graphene field effect phototransistors
fabricated on undoped semiconductor (SiC) substrates. The photodetection
mechanism relies on the high sensitivity of the graphene conductivity to the
local change of the electric field that can result from the photo-excited
charge carriers produced in the back-gated semiconductor substrate. We also
modeled the device and simulated its operation using the finite element method
to validate the existence of the field induced photoresponse mechanism and
study its properties. Our graphene phototransistor possesses a room-temperature
photoresponsivity as high as ~ 7.4 A/W, higher than the required
photoresponsivity (1 A/W) in most practical applications. The light
power-dependent photocurrent and photoresponsivity can be tuned by the
source-drain bias voltage and back-gate voltage. Graphene phototransistors
based on this simple and generic architecture can be fabricated by depositing
graphene on a variety of undoped substrates, and are attractive for many
applications in which photodetection or radiation detection is sought.
