Non-volatile semiconductor memory device
First Claim
1. A non-volatile semiconductor memory device, comprising:
- a memory cell array composed of a plurality of electrically data writable and erasable non-volatile memory cells arranged in a matrix pattern, each of the memory cells being a transistor having a source, a drain, a floating gate and a control gate;
data being written in each memory cell by injecting electrons into the floating gate thereof and erased from each memory cell by extracting electrons from the floating gate thereof;
said drain of each memory cell being connected to a bit-line which receives data from the memory cell, a test of data erasure being achieved by checking whether the source-drain can be turned on or not when a predetermined voltage is kept applied to the control gate, turning on or not of said memory cell being checked based on the potential of the bit-line, the bit-line being connected to an I/O pad when the test of data erasure is achieved so that the potential of the bit-line may be measured external to the memory device; and
a source bias circuit for applying a positive bias voltage to the sources of the memory cells to be tested in the erasure test, to shift each threshold value of each memory cell in a forward direction thereof, said positive bias voltage being a variable voltage that is externally applied to the memory device.
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Abstract
When an erase voltage is applied to the sources of data erasable and rewritable memory cells each having a floating gate, the erasure characteristics of the memory cells can be improved by controlling the rise time of the erase voltage or by increasing the erase voltage stepwise. In test mode, no row lines are selected by a row decoder and further the sources of the respective memory cells are set to ground level. Under these conditions, in case there exists an overerased memory cell, this cell is turned on due to depletion, so that it is possible to detect the presence of the overerased memory cell on the basis of change in potential of the column line connected to this turned on memory cell. A differential amplifier is used to detect the change in potential of the column line. In the test mode, the potential of the column lines is compared with a reference potential applied to a dummy column line, and a source bias generating circuit applies a test potential suitable for test to the respective sources of the cells, to shift the threshold level of the respective cells in a positive direction, for instance. By applying this test potential to the cells, it is possible to detect the pseudo-threshold level shifted in the positive direction; that is, to detect the overerased status of the memory cell more properly. Further, the erasure is effected until the threshold level of a memory cell of the highest erasure speed reaches a predetermined level, irrespective of the threshold distribution width of the memory cells, thus realizing a higher speed access to the device of narrower threshold distribution width, as compared with the conventional device.
42 Citations
15 Claims
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1. A non-volatile semiconductor memory device, comprising:
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a memory cell array composed of a plurality of electrically data writable and erasable non-volatile memory cells arranged in a matrix pattern, each of the memory cells being a transistor having a source, a drain, a floating gate and a control gate;
data being written in each memory cell by injecting electrons into the floating gate thereof and erased from each memory cell by extracting electrons from the floating gate thereof;
said drain of each memory cell being connected to a bit-line which receives data from the memory cell, a test of data erasure being achieved by checking whether the source-drain can be turned on or not when a predetermined voltage is kept applied to the control gate, turning on or not of said memory cell being checked based on the potential of the bit-line, the bit-line being connected to an I/O pad when the test of data erasure is achieved so that the potential of the bit-line may be measured external to the memory device; anda source bias circuit for applying a positive bias voltage to the sources of the memory cells to be tested in the erasure test, to shift each threshold value of each memory cell in a forward direction thereof, said positive bias voltage being a variable voltage that is externally applied to the memory device. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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Specification