The electrical properties of metal-insulator-semiconductor (MIS) devices based on ONO (SiO2-Si3N4-SiO2) structures fabricated on n-type 4H-SiC (0001) epilayers have been investigated. Three different combinations of low-pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD) and thermal oxidations (TO) in N2O and wet oxygen H2O:O-2 were studied for the formation of the ONO stack. In addition, the influence of the thickness of SiO2 and Si3N4 layers were considered and recommendations for optimal ONO structure are given. Oxide characterization tests and reliability investigations have been performed at room and high temperatures. This comparative study resulted in the development of ONO structures describing low oxide/near interface/interface defects and high reliability of the devices even at high temperature. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3556119] All rights reserved.