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In this letter, we report successful fabrication of germanium n-MOSFETs on lightly doped Ge substrates with a thin HfO2 dielectric (equivalent oxide thickness ∼10.8 Å) and TaN gate electrode. The highest peak mobility (330 cm2/V·s) and saturated drive current (130 μA/sq at Vg--Vt=1.5 V) have been demonstrated for n-channel bulk Ge MOSFETs with an ultrathin dielectric. As compared to Si control devices, 2.5× enhancement of peak mobility has been achieved. The poor performance of Ge n-MOSFET devices reported recently and its mechanism have been investigated. Impurity induced structural defects are believed to be responsible for the severe degradation.