High-Mobility n-MOSFET option with InGaAs channels are of intense interests. As the well-known interfacial trap (Dit) problem appears now contained, new challenges are emerging from above the interface. The evidence of oxide border traps (BT) in high-k dielectrics and its effect on the on-state performance of InGaAs n-MOSFETs are presented in this study through combined CV dispersion and AC transconductance analyses. Frequency dispersion in CV measurements can be associated with conductive losses of the MOS system. At different gate bias the conductive loss due to events such as minority carrier generation-recombination, trapping/de-trapping at the dielectric/III-V interface and within the dielectrics can be observed. By carefully examine the multi-frequency CV and GV response we can establish the conductance spectroscopy of the MOS device. Frequency dispersion in strong accumulation can be the result of oxide trapping. By measuring the device at different temperatures we have observed strong temperature dependence in CV dispersion at accumulation. The AC transconductance (AC-gm) measurement resembles the conventional CV measurement except for the additional small drain bias applied to inject carriers across the channel. This small but crucial difference allows one to examine the effect of oxide border traps on carrier transport. Clear frequency dispersion is observed on both the CV and AC-gm response of the InGaAs MOSFET. The increase in AC frequency reduces both the trapping time constant and the charging of the border traps, resulting in lower capacitance and yet higher peak transconductance. The amount of frequency dispersion on the AC-gm curves reflects the border trap density close to the oxide-semiconductor interface and clearly shows the impact of the traps on carrier transport.