1. Field of the Invention
The present invention relates to a semiconductor memory device such as NAND-cell, NOR-cell, DINOR (Divided bit line NOR)-cell and AND-cell type EEPROMs, and more particularly to a semiconductor memory device including an improved sense amplifier of the current sense type.
2. Description of the Related Art
NAND-type flash memories have faced increases in use in handling massive data such as images and motion pictures in mobile devices and so forth and sharp rises in demand. A flash memory utilizes a multivalue technology that enables one memory cell to store 2-bit or more information, thereby storing a larger amount of information in a smaller chip area. The NAND-type has a NAND string structure of serially connected memory cells, which results in small cell current. Therefore, it is required to sense the small cell current accurately.
A sense amplifier in a semiconductor memory device such as the flash memory basically senses the presence/absence of or the magnitude of the cell current flowing in accordance with data in a memory cell to determine the value of the data. The sense amplifier is usually connected to a bit line (data line) connected to a plurality of memory cells, and the sense system is roughly divided into the voltage sense type and the current sense type.
The sense amplifier of the voltage sense type precharges a bit line isolated from the memory cells up to a certain voltage, then discharges the bit line through a selected memory cell, and senses the discharged state of the bit line at a sense node connected to the bit line. On sensing data, the bit line is isolated from the current source load to sense the bit line voltage determined by the cell data.
The sense amplifier of the current sense type, on the other hand, supplies a read current flowing in the memory cell via the bit line to sense data. Also in this case, the cell data determines the bit line voltage, and finally data determination at the sense node connected to the bit line is executed by sensing a difference in voltage on the sense node based on the difference in cell current.
The sense amplifier of the voltage sense type and the sense amplifier of the current sense type have the following advantages and disadvantages. The voltage sense type utilizes charging and discharging the bit line and accordingly requires less power consumption. In a massive memory having a larger bit line capacity, though, charging and discharging consumes time, which causes difficulty in fast sensing. In addition, the bit line voltage is changed with relatively large amplitude in accordance with cell data, which causes a problem associated with noises between adjacent bit lines.
The sense amplifier of the current sense type, to the contrary, supplies a read current flowing in the memory cell via the bit line to sense data, thereby achieving fast sensing. A clamp transistor (pre-sense amplifier) is arranged between the bit line and a sense node to suppress the amplitude of the bit line voltage, which therefore hardly causes a problem associated with noises between bit lines. Also in this case, bit lines are subjected to reading at intervals of one, and the bit lines not subjected to reading data are grounded and used as shield to preclude the mutual influence between bit lines on reading.
On the other hand, the bit line potential is always controlled and fixed at a constant voltage on sensing to preclude the mutual influence between adjacent bit lines to sense all bit lines in parallel as can be executed in a sense amplifier of the ABL (All Bit Line) type proposed (Patent document 1: JP 2006-500729T, paragraphs 0080-0088, FIG. 14).
The sense amplifier of the ABL type precharges an inner sense node and the bit line and then discharges the charge on the sense node to the bit line. When the potential on the sense node lowers to a certain value, current is supplied continuously in the bit line through a different path. The continuous current flowing in the bit line in this way fixes the potential on the bit line at a certain potential. The precharged voltage on the bit line and the variation in potential on the sense node vary depending on whether the memory cell connected to the bit line is an on-cell or an off-cell. Accordingly, by sensing the potential on the sense node, the data state in the memory cell can be read out.
In such the sense amplifier of the ABL type, on switching to the operation of releasing the charge precharged on the sense node to the bit line, the potential on the sense node lowers even if the selected cell connected to the bit line is an off-cell. Namely, when the selected cell connected to the bit line is an off-cell, the bit line can be charged completely by precharging ideally. Complete charging, however, consumes time and accordingly slight lack in charge may arise usually. Therefore, even if the selected cell is an off-cell, the sense node is discharged to the bit line to some extent on sensing. As a result, the difference in current between the on-cell and the off-cell decreases and the difference in potential appeared on the sense node also decreases, resulting in a reduction in sense margin.