In order to examine the peak carrier mobility reduction in ultrathin high-kappa based FETs, interface state density in HfSiON gated FETs (of EOT <2 nm) was studied experimentally using two high resolution techniques. Both techniques independently established similar interface charge density values for the devices studied. By further studying the Coulomb limited mobility in the high-kappa based FETs using a quantum mechanical approach, it is concluded that interface charge alone is insufficient to explain the peak mobility degradation observed, indicating that remote charge scattering. RCS, may be a significant mobility degradation factor in ultra thin HfSiON based FETs. (C) 2010 Elsevier Ltd. All rights reserved.