Semiconductor device having electrically erasable programmable read-only memory (EEPROM) and mask-ROM and method of fabricating the same

A semiconductor device including an EEPROM and a Mask-ROM transistor, and methods of fabricating and forming the same, where a device isolation layer may be formed at given regions of a semiconductor substrate to define a cell active region, and a Mask-ROM active region including a channel doped region therein. A channel doped region may be formed within the Mask-ROM active region, and a plurality of Mask-ROM gates may be formed that cross the channel doped region. A Mask-ROM gate insulating layer may be interposed between a Mask-ROM gate and the Mask-ROM active region, and the device isolation layer may have a surface adjacent to the channel doped region that is lower as compared to a surface of the device isolation layer that is not directly adjacent to the channel doped region.