Non-volatile read only memory and its manufacturing method
AssigneeOki Electric Industry Co., Ltd.
InventorsNoboru Egawa, Hitoshi Kokubun
Life of this patent
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Abstract
The mask ROM for storing quaternary data that enables a short turn around time, makes refining cell sizes simple, and that enables stable reading of data. Gaps are formed between word lines in the memory cell transistors and two n+ diffusion areas. n+ impurities are doped into these gaps in accordance with quaternary write data when data is written. A current runs between these diffusion areas only when one of these two areas into which impurities have been doped is used as a drain. Accordingly, quaternary data can be read by reading once when one diffusion area is a source and the other diffusion area is a drain and by reading again when the first diffusion area is used as a drain and the other as a source.
Claims10
A non-volatile Read Only Memory, comprising:
- memory cell transistors each of which is constituted such that the area between the gate electrode and one impurity area and the area between said gate electrode and the other impurity area take offset structure or non-offset structure in accordance with the value of stored data;
- a row selection line that applies a voltage to said gate electrodes in said memory cell transistors belonging to same row;
- a first column selection line that applies a voltage to said one impurity area in said memory cell transistors belonging to same column;
- a second column selection line that applies a voltage to said other impurity area in said memory cell transistors belonging to same column;
- and
- a reading circuit which applies a read electric potential to said row selection line selected, and reads said stored data by detecting the on/off status of said memory cell transistors when high electric potential is applied to said first column selection line and low electric potential is applied to said second column selection line, and by detecting the on/off status of said memory cell transistors when low electric potential is applied to said first column selection line and high electric potential is applied to said second column selection line.
Dependencies are inferred from the claim text.