We report photodiodes fabricated on Ge grown on Si substrate. The Ge growth was preceded by two SixGe1-x buffer layers. Dark current as low as 1.07μA was achieved at 10V reverse bias for 24μm-diam mesa devices. At 1.3μm wavelength, the responsivity was 0.37A/W at 0V and 0.57A/W when above 2V reverse bias. The 3dB bandwidth was 8.1GHz at a reverse bias of 10V.