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Ge photodiodes with ultra-high responsivities have been demonstrated with gold nanoparticle assistance. The responsivity can reach a value of 37.7 A/W, which corresponds to a gain of 76. Such a high responsivity originates from the amassment of photo-generated holes in Ge under the boundary of gold nanoparticles, which reduces the barrier for electron tunneling from metal to the hole-amassment Ge ring areas. The depletion layer thinning due to hole amassment is not only proved by band diagram simulation but also confirmed by capacitance measurement.