Enhancement-mode nMISFET's fabricated on 6H-SiC substrates exhibit remarkable reliability and durability at high temperatures. These devices incorporate ONO (oxide-nitride-oxide) gate dielectrics made by the JVD (Jet-Vapor Deposition) method. Projected operating lifetimes of over 200 years under an oxide field of 3 MV/cm, and over 10 years under a field of 3.6MV/cm, have been demonstrated at 450 degreesC. These values are believed to far exceed the best data reported to date.