1. Technical Field
The present disclosure relates to a semiconductor memory device, and particularly to a semiconductor memory device having an open bit line structure and a method of testing thereof.
2. Discussion of Related Art
A semiconductor memory device is used for storing data. A random access memory device (RAM) is a volatile memory device and is mainly used as a main memory device of a computer. A dynamic random access memory device (DRAM), which is a kind of volatile RAM, is comprised of memory cells. In general, a memory cell comprising a transistor and a capacitor may store information as a charge in the form of “1” or “0.” Capacitors included in memory cells are periodically refreshed, because charges stored in the capacitors may be lost as time goes by.
The memory cells of a DRAM are coupled to word lines and bit lines. When transistors included in the memory cells are turned on, data stored in the capacitors will be outputted or data of a bit line will be transferred to the capacitors.
A semiconductor memory device is classified into a semiconductor memory device having a folded bit line structure and a semiconductor memory device having an open bit line structure, according to the structure of the memory cell array. The memory cell array of a semiconductor memory device having an open bit line structure includes edge sub-arrays having dummy bit lines. The dummy bit lines are not coupled to sense amplifiers and data may not be stored in the memory cells coupled to the dummy bit lines.
FIG. 1 is a schematic diagram illustrating a memory core of a conventional semiconductor memory device having an open bit line structure.
Referring to FIG. 1, the memory core 100 of a semiconductor memory device includes edge sub-arrays 120 and 130, non-edge sub-arrays 110, and sense amplifiers 140-1 to 140-n and 150-1 to 150-n. The non-edge sub-arrays 110 include bit lines corresponding to horizontal lines and word lines corresponding to vertical lines, and a memory cell is located at each of the intersecting points 111 of a word line and a bit line. The edge sub-arrays 120 and 130 include word lines WL, bit lines BL and dummy bit lines DBL. A memory cell is located at each of the intersecting points 121 and 123 of a word line WL and a bit line BL, and at each of the intersecting points 121 and 123 of a word line WL and a dummy bit line DBL.
As shown in FIG. 1, the dummy bit lines DBL are not coupled to sense amplifiers. Therefore, data may not be sensed or read through the dummy bit lines DBL included in the edge sub-arrays 120 and 130. Accordingly, it is not easy to input data to the dummy bit lines DBL and sense the data from the dummy bit lines DBL in a test mode.
FIG. 2 is a schematic diagram illustrating a conventional semiconductor memory device having sense amplifier circuits and memory cell arrays. Referring to FIG. 2, the semiconductor memory device 170 includes a bit line BL, bit line bar BLB, an equalizer 173 coupled between the bit line BL and the bit line bar BLB, a p-type sense amplifier 174, an n-type sense amplifier 175 and a column selecting circuit 176. Further, the semiconductor memory device 170 includes a memory cell 171 coupled between a word line WLi and a bit line BL, and a memory cell 172 coupled between a word line WLj and a bit line bar BLB.
The memory cell 171 includes a cell switch MN1 and a cell capacitor C1. The bit line BL has capacitance CBL and the bit line bar BLB has capacitance CBLB. When data stored in the cell capacitor C1 is outputted to the bit line BL, charge sharing is generated between the cell capacitor C1 and the bit line BL, which has the capacitance CBL.
FIG. 3 is a schematic diagram illustrating a memory core of another conventional semiconductor memory device having an open bit line structure, and is disclosed in U.S. Pat. No. 6,535,439. The semiconductor memory device in FIG. 3 includes sense amplifiers 260-1 to 260-n and 270-1 to 270-n to more accurately test the edge sub-arrays included in the semiconductor memory device.
The semiconductor memory device in FIG. 3, however, may occupy a large area in the semiconductor integrated circuit, because each of the dummy bit lines is coupled to each of the sense amplifiers.