  We propose a hybrid device consisting of a graphene/silicon (Gr/Si) Schottky
diode in parallel with a Gr/SiO2/Si capacitor for high-performance
photodetection. The device, fabricated by transfer of commercial graphene on
low-doped n-type Si substrate, achieves a photoresponse as high as 3 AW^(-1)
and a normalized detectivity higher than 3.5 10^12 cmHz^(1/2) W^(-1) in the
visible range. The device exhibits a photocurrent exceeding the forward
current, because photo-generated minority carriers, accumulated at Si/SiO2
interface of the Gr/SiO2/Si capacitor, diffuse to the Gr/Si junction. We show
that the same mechanism, when due to thermally generated carriers, although
usually neglected or disregarded, causes the increased leakage often measured
in Gr/Si heterojunctions. At room temperature, we measure a zero-bias Schottky
barrier height of 0.52 eV, as well as an effective Richardson constant A**=4
10^(-5) Acm^(-2) K^(-2) and an ideality factor n=3.6, explained by a thin (<
1nm) oxide layer at the Gr/Si interface.
