We describe, model and demonstrate a tunable micro-ring resonator integrated monolithically with a photodiode in a silicon waveguide device. The photodiode is made sensitive to wavelengths at and around 1550nm via the introduction of lattice damage through selective ion implantation. The ring resonator enhances detector responsivity in a 60 mum long waveguide photodiode such that it is 0.14 A/W at -10Vbias with less than 0.2 nA leakage current. The device is tunable such that resonance (and thus detection) can be achieved at any wavelength from 1510 - 1600 nm. We also demonstrate use of the device as a digital switch with integrated power monitoring, 20 dB extinction, and no optical power tapped from the output path to the photodiode. A theoretical description suggests that for a critically coupled resonator where the round trip loss is dominated by the excess defects used to mediate detection, the maximum responsivity is independent of device length. This leads to the possibility of extremely small detector geometries in silicon photonics with no requirement for the use of III-V materials or germanium.