An improved method of making a semiconductor device such as an N-channel, double level poly, MOS read only memory or ROM array is provided; the array is of very dense structure and may be electrically programmable by floating gates which are interposed between the gate oxide and control gates formed by polycrystalline silicon or metal row address lines. The electrical programming of the cells is accomplished by applying selected voltages to the source, drain, control gate and substrate. The very dense array results from a simplified manufacturing process generally compatible with standard N-channel silicon gate technology. Parallel strips of gate oxide, polycrystalline silicon, and nitride (functioning as an oxidation mask) are created in one mask step before field oxide is grown, then a perpendicular pattern of conductive strips is etched using a second mask step.