We present high-sensitivity photoreceivers based on a vertical- illumination-type 100% Ge-on-Si p-i-n photodetectors (PDs), which operate up to 50 Gb/s with high responsivity. A butterfly-packaged photoreceiver using a Ge PD with 3-dB bandwidth (f(-3dB)) of 29 GHz demonstrates the sensitivities of -10.15 dBm for 40 Gb/s data rate and -9.47 dBm for 43 Gb/s data rate, at BER of 10(-12) and λ ~1550 nm. Also a photoreceiver based on a Ge PD with f(-3dB)~19 GHz shows -14.14 dBm sensitivity at 25 Gb/s operation. These results prove the high performance levels of vertical-illumination type Ge PDs ready for practical high-speed network applications.