We report a vertical p-i-n thin-film germanium photodetector integrated on 3microm thick large core silicon-on-insulator (SOI) waveguides. The device demonstrates very high external responsivity due to the low fiber coupling loss to the large core waveguides. The germanium width and thickness are carefully designed to achieve high responsivity yet retain high-speed performance. Even with fiber coupling loss included, the device has demonstrated greater than 0.7A/W external responsivity at 1550nm for TM polarization and 0.5A/W for TE polarization. A low dark current of 0.2microA at -0.5V bias is reported. 3dB bandwidths of 12GHz and 8.3GHz at -2.5V bias are also reported for 100microm and 200microm long devices, respectively. The device can cover the communication wavelength spectrum up to 1620nm with a relatively flat responsivity of >0.5A/W. Further studies suggest that with a modified design the device is capable of achieving 1A/W external responsivity for both TE and TM polarizations and greater than 30GHz bandwidth.