Semiconductor memory devices are configured to store data therein. Semiconductor memory devices are generally divided into volatile and nonvolatile types. Nonvolatile memory devices retain data stored therein even without power supply, while volatile memory devices lose their data when power supplies are interrupted or suspended.
With data retention capability in low power, nonvolatile memory devices are nowadays regarded as useful storage media for portable apparatuses. Nonvolatile memories include various kinds, e.g., flash memories, phase-change random access memories (PRAMs), ferroelectric RAMs (FeRAMs), magnetic RAMs (MRAMs), and so on.
Flash memories are suitable for high integration density, widely used over mobile systems on the merits of nonvolatiles. A flash memory is organized by including pluralities of memory blocks.
MRAMs, like hard disks, store data by means of magnetic properties. MRAMs employ ferromagnetic tunnel magneto resistance (TMR) devices for storing data.
PRAMs are operable by using thin film materials such as chalcogenide alloys (e.g., Ge2Sb2Te5; GST), which are used in CD-ROMs or DVD-RAMs. Resistance of such a chalcogenide alloy becomes larger in an amorphous state, but smaller in a crystalline state. Thus, data ‘1’ or ‘0’ is stored therein by sensing a level of resistance from the chalcogenide alloy film.
FeRAMs utilize ferroelectric materials for storing data. In a ferroelectric material, a polarization is directional by a voltage applied thereto. Therefore, data is distinguished with reference to a polarized orientation in a FeRAM.
In the meantime, there is a limit to change times of data stored in such a semiconductor memory device because it is gradually becomes worn out by repetitive writing and erasing operations. With an increase in wearing degree, it takes a longer time to change (e.g., write or erase) data, which raises the probability of malfunctions while changing data. Consequently, a unit cell of the semiconductor memory device could be deprived of its own data storage capability.
In a semiconductor memory device, wearability against the writing and erasing operations is typically represented as an index of endurance. Endurance of the semiconductor memory device is determined by operable (or usable) writing or erasing times without malfunctions. Endurance of the semiconductor memory device is usually rated up to tens of thousands to millions in usable times.
In order to lengthen a lifetime (or wearability) of a semiconductor memory device, it requires uniformity of data change events (e.g., writing or erasing). If operations for changing data states are concentrated on a specific area of a semiconductor memory device, it eventually causes a great deal of decadence to a lifetime thereof. But if data changing operations are conducted uniformly over cell areas of a semiconductor memory device, it may extend its lifetime significantly.
There have been proposed many ways of leveling or managing wearability of semiconductor memory devices such as flash memories. However, different from other kinds of memories, flash memories are inoperable in an overwriting mode. PRAMs, MRAM, or FeRAMs would be degraded in performance if the wear-leveling schemes used for the flash memories were applied thereto, because those memories are operable in the overwriting mode. Therefore, there is a need of providing a wear-leveling scheme for a semiconductor memory system that is operable in the overwriting mode.