A detailed comparison of high-k/metal gate stacks fabricated on the (100)-and (110)-oriented substrates has been made. We have verified that there is no structural difference between the gate stacks on the two substrates. The orientation independence of our gate stacks with the highquality high-k/silicon interface for both substrates reveals the intrinsic effects of the crystal orientation on the high-k/metal gate devices. This is the first direct evidence that the inversion carrier quantization dependency on the crystal orientations dominates the total gate capacitance in the high-k/metal gate system. The analysis of the electron and hole mobilities highlights the fact that the scattering mechanisms inherent in the highk/meta gate stacks limit the mobility enhancement and reduction driven by the effective mass change. No significant difference in the threshold voltage instability under the positive or negative gate bias stress is observed for the two substrates. Therefore, the optimized high-k/metal gate stacks enable us to gain certain benefits from the limited modulation of the mobility without sacrificing the stability of the threshold voltage. © 2011 The Japan Society of Applied Physics.