Electrically alterable read-only memory cell with graded energy band-gap insulator

Low voltage alterability and excellent memory retention have been obtained with a novel EAROM cell that has a graded energy band-gap film as the first insulator of a Floating-Gate type memory. The graded energy band-gap insulator can enhance charge injection without deteriorating memory retention, because the energy band-gap is narrowed only at the silicon substrate interface. A graded energy band-gap insulator has been realized by thermally oxidizing the surface of a very thin silicon nitride film grown by direct thermal nitridation of a silicon substrate. A fabricated EAROM cell has shown that it can be programmed by a single positive supply of less than 12 V and it has excellent memory retention.