The mobility degradation mechanisms of n-channel metal-oxide-semiconductor field effect transistors with ZrO2 and Sm2O3 gate dielectrics were studied. The temperature dependence of device characteristics was studied in the temperature range from 11 to 300 K. n-channel metal-oxide-semiconductor field effect transistors (MOSFETs) with SiO2 gate dielectric are used as reference. The electron mobility of ZrO2-gated n-MOSFETs is limited by Coulomb scattering. The extra source of Coulomb scattering is attributed to additional oxide trapped charges. The electron mobility in Sm2O3-gated n-MOSFETs is limited by phonon scattering. The extra source for phonon scattering is attributed to soft optical phonons in Sm2O3.