An asymmetrical lightly doped source cell for virtual ground high-density EPROMs

An EPROM cell structure with asymmetrically doped source and drain junctions, which is suitable for a virtual ground array architecture, is described. The asymmetric cell can eliminate a write disturb to an adjacent cell, which is inherent in virtual ground designs, in the write mode. In the read mode, the cell provides higher read current and substantial soft-write lifetime, along with optimization of the lightly doped n⁺ region concentration. Virtual ground designs using the cell have been implemented. A planarized process to improve manufacturability is discussed. These reliable, scalable cell and array structures are judged promising for the next generation of high-density EPROMs