(color online) Integrated graphene/silicon heterostructure photodetector. (a) Schematic of graphene/silicon heterostructure waveguide photodetector. The photodetector is formed by a graphene/silicon heterojunction on an SOI waveguide. Electrodes and focusing sub-wavelength grating are also demonstrated (from Ref. ^[55]). (b) Responsivity of graphene silicon and reference metal silicon Schottky photodetector, showing the photogain under negative bias conditions. Colored solid lines show a fit of the bias-dependent responsivity based on combined thermionic-field emission and avalanche multiplication process. (c) SEM micrograph of graphene silicon Schottky photo-detector. (Left) Graphene is coupled to a waveguide. (Right) Layout of the complete device (from Ref. ^[56]).