  We measure the channel potential of a graphene transistor using a scanning
photocurrent imaging technique. We show that at a certain gate bias, the impact
of the metal on the channel potential profile extends into the channel for more
than 1/3 of the total channel length from both source and drain sides, hence
most of the channel is affected by the metal. The potential barrier between the
metal controlled graphene and bulk graphene channel is also measured at various
gate biases. As the gate bias exceeds the Dirac point voltage, VDirac, the
original p-type graphene channel turns into a p-n-p channel. When light is
focused on the p-n junctions, an impressive external responsivity of 0.001 A/W
is achieved, given that only a single layer of atoms are involved in photon
detection.
