A 1024 bit N-channel MOS read-write memory chip

The Cogar 1024-bit MOS read-write memory chip is fully decoded, has a 100-ns access time, and is only 125 × 125 mils in size. This small size is made possible by the use of a 4-device memory cell and dimensional tolerances as small as 0.15 mil. The chips are fabricated from a 2.25-inch-diameter wafer that consists of a low-resistivity p-type substrate covered by a high-resistivity p-type epitaxial layer. This epitaxial layer provides not only for precise control of surface impurity concentration, but also provides means for a reach-through type of gate protection device. In addition, the low-resistivity substrate tends to minimize unwanted noise voltages due to pulse currents flowing through the substrate.