The dynamic random access memory (DRAM) market shows a growing need for DRAM products that consume very little power. DRAM products that consume very little power in a standby mode in a mobile application, such as in a cellular phone or other portable electronic device, are especially in demand. A DRAM memory cell needs to be refreshed during standby mode to maintain the information stored in the memory cell. An increase in the time between refreshes of memory cells reduces the amount of power consumed by the memory cells in a standby mode. The time between refreshes of memory cells is limited by the data retention time of the memory cells. Since the time between refreshes is the same for all memory cells in an array of memory cells, the refresh limit is based on the data retention time not of the average memory cell in the array of memory cells, but of the worst memory cell in the array of memory cells.
Using two memory cells to store a single bit of data significantly increases the data retention time. The data retention time increases since there is a high probability that a bad memory cell will be combined with an average memory cell. The resulting combined data retention time will then be much closer to the average data retention time. It is advantageous, however, to test and repair a memory using a single cell configuration to find and replace the worst cells based on single cell test results so that the worst cells are identified more easily and the redundancy in the array of memory cells is used more effectively.
For low power and/or battery powered applications, such as cellular phones and other portable electronic devices, memories that conserve power are desired. These memories may include dynamic random access memories (DRAMs), synchronous dynamic random access memories (SDRAMs), double data rate synchronous dynamic random access memories (DDR SDRAMs), double data rate II synchronous dynamic random access memories (DDR2 SDRAMs), pseudo static random access memories (PSRAMs), cellular random access memories (Cellular RAMs), and mobile random access memories (Mobile RAMs).