A SiC die with Os and/or W/WC/TiC contacts and metal conductors is encapsulated either alone or on a ceramic substrate using a borosilicate (BSG) glass that is formed at a temperature well below upper device operating temperature limits but serves as a stable protective layer above the operating temperature (over 1000.degree. C., preferably >1200.degree. C.). The glass is preferably 30-50% B.sub.2O.sub.3/70-50% SiO.sub.2, formed by reacting a mixed powder, slurry or paste of the components at 460.degree.-1000.degree. C. preferably about 700.degree. C. The die can be mounted on the ceramic substrate using the BSG as an adhesive. Metal conductors on the ceramic substrate are also protected by the BSG. The preferred ceramic substrate is AIN but SiC/AIN or Al.sub.2 0.sub.3 can be used.