Graphene-based photodetectors have attracted strong interest for their exceptional physical properties, which include an ultrafast response across a broad spectrum, a strong electron-electron interaction and photocarrier multiplication. However, the weak optical absorption of graphene limits its photoresponsivity. To address this, graphene has been integrated into nanocavities, microcavities and plasmon resonators, but these approaches restrict photodetection to narrow bands. Hybrid graphene-quantum dot architectures can greatly improve responsivity, but at the cost of response speed. Here, we demonstrate a waveguide-integrated graphene photodetector that simultaneously exhibits high responsivity, high speed and broad spectral bandwidth. Using a metal-doped graphene junction coupled evanescently to the waveguide, the detector achieves a photoresponsivity exceeding 0.1 A W -1 together with a nearly uniform response between 1,450 and 1,590 nm. Under zero-bias operation, we demonstrate response rates exceeding 20 GHz and an instrumentation-limited 12 Gbit s -1 optical data link. © 2013 Macmillan Publishers Limited. All rights reserved.