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Mechanisms of the mobility degradation in high normal field (or Ns) in Ge pand n-metal-oxide-semiconductor field-effect transistors (MOSFETs) with plasma postoxidation GeOx/Ge MOS interfaces: 1) carrier trapping due to surface states; 2) surface roughness scattering; and 3) electron transfer into the Δ valleys with the high effective mass, have been systematically investigated. It is confirmed that the existence of surface states inside the valence band and the conduction band of Ge results in over estimation of the mobile inversion carrier concentration and the rapid reduction of the effective mobility. It is also found that the Hall hole and electron mobility in high Ns region agree well with the theoretical surface roughness limited mobility, indicating that the high Ns mobility in Ge MOSFETs are still significantly limited by surface roughness scattering. On the other hand, any evidence of the carrier repopulation into the subband with low mobility has not been experimentally identified in both Ge pand n-MOSFETs yet.