Nonvolatile semiconductor memory
View Patent ↗Each nonvolatile memory cell transistor has such directivities that a current flows only from the drain to the source and that charge is exchangeable only at the source. The source of one of a pair of memory cell transistors connected to each word line is connected to the drain of the other memory cell transistor, and the drain of the one memory cell transistor is connected to the source of the other. During a data rewrite operation, reverse voltages are applied to the sources and drains of the pair of memory cell transistors. Because of the directivities of each memory cell transistor, charge is exchanged with a charge accumulation layer only in the source region. This makes the data rewritable in only one of the pair of memory cell transistors. As a result, data is rewritable on a memory cell basis without increasing the memory cell size.
1. A nonvolatile semiconductor memory comprising:
a plurality of nonvolatile memory cell transistors that are arranged in matrix form and have directivities that a current flows from a drain to a source only and that charge is exchangeable only at a source;
a plurality of word lines that are connected to respective control gate groups of the memory cell transistors that are arranged in one direction;
a first bit line that is connected to a source of one of a pair of the memory cell transistors and to a drain of the other of the pair of memory cell transistors, said pair of memory cell transistors being adjacent to each other and connected to one of said word lines; and
a second bit line that is connected to a drain of the one of said pair of memory cell transistors and a source of the other of said pair of memory cell transistors.
2. The nonvolatile semiconductor memory according to claim 1 , wherein:
the memory cell transistors connected to each of said word lines have the drains arranged such that the drains are directed to a same side;
the memory cell transistors arranged in a direction perpendicular to said word lines have the drains and sources arranged alternately such that the drains and sources are directed to reverse sides; and
said first and second bit lines are wired in a twisted pair form in a direction perpendicular to said word lines.
3. The nonvolatile semiconductor memory according to claim 2 , wherein:
said first and second bit lines are wired by use of a first wiring layer and a second wiring layer; and
said first and second wiring layers are switched at a drain node to which drains facing each other are connected and at a source node to which sources facing each other are connected.
4. The nonvolatile semiconductor memory according to claim 1 , wherein:
the memory cell transistors connected to each of said word lines have the drains arranged such that the drains are directed to a same side;
the memory cell transistors arranged in a direction perpendicular to said word lines have the drains and sources arranged alternately such that the drains and sources are directed to reverse sides; and
said first and second bit lines are wired in a zigzag in a direction perpendicular to said word lines.
5. The nonvolatile semiconductor memory according to claim 4 , wherein
said first and second bit lines are wired by use of only a first wiring layer.
6. The nonvolatile semiconductor memory according to claim 1 , further comprising
an erase control circuit setting a threshold voltage of the memory cell transistor at a predetermined positive value after setting at a negative value, in an erase operation for erasing data stored in the memory cell transistor, the predetermined positive value indicating an erased state.
7. The nonvolatile semiconductor memory according to claim 1 , wherein
said memory cell transistors have an offset region between said drains and control gates in order to realize said directivities.
8. The nonvolatile semiconductor memory according to claim 1 , further comprising
a refresh control circuit performing, in response to a refresh request, a refresh operation for reading data held in the memory cell transistor and rewriting the read data.
9. The nonvolatile semiconductor memory according to claim 8 , further comprising
a refresh request circuit issuing the refresh request in accordance with a number of times the memory cell transistor is accessed.
10. The nonvolatile semiconductor memory according to claim 9 , further comprising
an access counter counting the number of times the memory cell transistor is accessed.