Mobility of fully depleted silicon-on-insulator MOSFETs with ultrathin body (8 nm) and buried oxide (10 nm) and with equivalent oxide thickness (EOT) of about 0.8 nm processed with HfO2 was investigated and compared with a device with an SiON-based dielectric. Under positive back-gate bias, we observed a maximum mobility improvement of approximately 150% for the HfO2 device; however, this maximum mobility is 20% lower than the mobility of the SiON device. Using a temperature analysis and a careful study of the back-and front-channel activations, we found that this improvement is explained by one channel located far from both interfaces. However, we also deduced that in this region, the mobility is strongly dependent of the transversal field. A larger field consistent with a lower EOT and larger charge defects explains the cause of the mobility degradation for the HfO2 device. Furthermore, a lower coupling factor for this device enhances this degradation.