Large contact resistance due to Fermi level pinning effect at the interface between metal and Ge strongly restricts the 3 dB bandwidth of Ge photodetectors. In this paper, the Ge PIN photodetectors fabricated on silicon-on-insulator substrates, respectively, with Al and Al/TaN electrodes are comparatively studied. It is found that 3 dB bandwidth of photodetector with 24 μm mesa diameter using an Al/TaN stack electrode is improved by four times more than that of the same structure Ge PIN photodetector using an Al electrode under -1 V bias at 1.55 μ. In addition, the dark current is reduced by one order of magnitude, and optical responsivity is enhanced by two times. These results suggest that a thin metallic TaN layer as an electrode can effectively passivate the Ge surface and alleviate the Fermi-level pinning effect, thus reducing the contact resistance and the recombination current at the interface. TaN can be considered as a promising electrode material for Ge device applications.