The high-temperature stability of lanthanum silicate gate dielectric metal-insulator-semiconductor (MIS) devices with either Ta or TaN electrodes has been studied. After a 1000 degrees C, 10 s rapid thermal annealing (RTA) treatment, devices with Ta gate metal undergo an equivalent oxide thickness (EOT) increase from 0.62 to 1.57 nm or higher, while devices with TaN as the gate electrode experience an EOT increase from 0.62 to only 1.12 nm. An EOT less than 1.0 nm is achieved after a 5 s 1000 degrees C RTA, with a corresponding gate leakage of 0.1 A/cm(2). Medium-energy ion scattering and X-ray diffraction (XRD) analysis reveal that the Ta gate metal undergoes a phase change due to reaction with N-2 above 800 degrees C, while for TaN no change in the XRD spectrum is detected. Interface state defect densities and leakage currents are reduced after the high-temperature processing. Results reveal the importance of the entire gate stack design and processing in obtaining good device properties.