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This letter presents the device performance of scaled thin-film-Ge lateral PIN photodetectors integrated on a Si waveguide. The photodetectors are with closely spaced p+/n+ regions (0.8 mum) on a Ge region with short length (5-20 mum) and narrow width (2.4 mum). Though with a thin Ge layer (~220 nm including bottom SiGe buffer), light is evanescent-coupled from the Si waveguide effectively to the overlying Ge detector. The device exhibits f3 dB bandwidth of 18 GHz with external responsivity of 0.13 A/W for 1550 nm at -1 V. Considering the coupling loss and waveguide loss, the internal responsivity is as high as 0.65 A/W. It is shown that with increasing detector length, the devices' internal quantum efficiency can be improved to ~90% and by suppressing parasitic effects, speed can be boosted further towards several tens of gigahertz.