FLOATING GATE TYPE NONVOLATILE MOS SEMICONDUCTOR MEMORY DEVICE

PURPOSE:To make writing characteristics excellent and to prevent soft writing, by separately arranging a write only transistor, which commonly posseses a floating gate and is easy to write, and a read only transistor, which is hard to write, on one memory cell. CONSTITUTION:A source part 20 is commonly used for both writing and reading. A drain part is divided into a write only part 21 and a read only part 22 by a field oxide silicon film 13. The interval between the source part 20 on the write only side and the drain part 21 is made to be, e.g., as short as about 3.5mum, and electrons are injected. Since a floating gate is in common, the electrons are distributed on the read only side. Meanwhile, the interval between the source part 20 on the read only side is made to be, e.g., as long as about 5mum, and electric field strength between both parts 20 and 22 is made weak. Thus, soft writing can be reduced to the minimum.