The monolithic integration of power-efficient optoelectronic devices with CMOS circuits is critical for future on-chip optical communication. In such platforms, ultra-high-speed photodetectors operating at datacom wavelengths are essential to convert optical signals in the electrical domain. Here, we demonstrate ultra-compact high-speed III-V/Si photodetectors integrated on silicon photonics and exclusively fabricated with processes and materials compatible with CMOS foundries. The sub-femtofarad capacitance, high responsivity photodetectors demonstrate a bandwidth around 65 GHz and data reception at 100 GBd OOK. Their thickness and efficiency enable an integration in the back-end-of-line without using an amplifier, further reducing the power consumption of the link.