  Graphene integrated photonics provides several advantages over conventional
Si photonics. Single layer graphene (SLG) enables fast, broadband, and
energy-efficient electro-optic modulators, optical switches and photodetectors
(GPDs), and is compatible with any optical waveguide. The last major barrier to
SLG-based optical receivers lies in the low responsivity - electrical output
per optical input - of GPDs compared to conventional PDs. Here we overcome this
shortfall by integrating a photo-thermoelectric GPD with a Si microring
resonator. Under critical coupling, we achieve $>$90% light absorption in a
$\sim$6 $\mu$m SLG channel along the Si waveguide. Exploiting the
cavity-enhanced light-matter interaction, causing carriers in SLG to reach
$\sim$400 K for an input power of $\sim$0.6 mW, we get a voltage responsivity
$\sim$90 V/W, demonstrating the feasibility of our approach. Our device is
capable of detecting data rates up to 20 Gbit/s, with a receiver sensitivity
enabling it to operate at a 10$^{-9}$ bit-error rate, on par with mature
semiconductor technology. The natural generation of a voltage rather than a
current, removes the need for transimpedance amplification, with a reduction of
the energy-per-bit cost and foot-print, when compared to a traditional
semiconductor-based receiver.
