Semiconductor devices and methods for forming a semiconductor device are disclosed. The method includes providing a substrate prepared with a memory cell region. A first gate structure is formed on the memory cell region. An isolation layer is formed on the substrate and over the first gate structure. A second gate structure is formed adjacent to and separated from the first gate structure by the isolation layer. The first and second gate structures are processed to form at least one split gate structure with first and second adjacent gates. Asymmetrical source and drain regions are provided adjacent to first and second sides of the split gate structure.