1. Field of the Invention
The invention relates generally to the field of electronic circuits and more particularly to designs for SRAM memory cells that provide improved stability in comparison to conventional designs.
2. Related Art
Computer systems and other devices typically need to have means for storing information.
These means may include persistent storage devices for large amounts of data, as well as smaller memory systems for storing data that the computer or other device is currently using. The memory systems for storing currently used data include both read-only memory (ROM) and random access memory (RAM.)
RAM is typically used as the working memory of a device. RAM is used by devices to store data that needs to be accessible by a processor, and also needs to be modifiable. That is, the data can be changed. By contrast, data stored in a ROM cannot be modified, but can only be read. There is a great demand for RAM in computers and other electronic devices because the more RAM a device has, the more data can be readily accessible to the device's processor. For example, in a computer, the availability of more RAM enables the computer to execute more (or larger) software applications without having to swap data between RAM and a persistent storage device, such as a hard disc drive.
There are various different types of RAM. For example, dynamic RAM, or DRAM, has often been used in computers. The “dynamic” aspect of DRAM refers to the fact that DRAM memory cells need to be periodically refreshed in order to maintain the data which is stored in the cells. If the DRAM cells are not refreshed, the data will be lost. Static RAM, or SRAM, is another type of memory that is often used in computers. That “static” aspect of SRAM refers to the fact that SRAM cells do not have to be refreshed in the same manner as DRAM cells.
SRAM memory has a number of advantages over DRAM memory. As noted above, SRAM cells do not have to be refreshed in order to maintain the data that is stored in them. Additionally, SRAM is typically much faster than DRAM. For example, typical SRAM cells may have access times of about less than 1 nanosecond, while DRAM cells may have access times closer to 60 nanoseconds. Further, SRAM memories do not require pauses between accesses, so the cycle time to access SRAM cells is typically much shorter than the cycle time for accessing DRAM cells.
Although it has a number of advantages, SRAM memory also has some disadvantages. For instance, SRAM is typically much more expensive to manufacture than DRAM. Because the cost of SRAM memory is much higher than DRAM, it is common for SRAM memory to be used as a memory cache, while DRAM is used for a processor's main memory.
SRAM memory cells may also be unstable. That is, the data in the cells may actually be corrupted when the cells are read. This problem arises from the fact that SRAM cells are read by coupling the cells to pre-charged bit lines and allowing the cells to pull down these bit lines. In other words, a higher voltage on the bit line is coupled to a lower voltage in the SRAM cell, causing the bit line voltage to drop and the SRAM cell voltage to rise. The voltage drop on the bit line is detected and amplified to provide the data for the associated processor.
The voltage rise in the SRAM cell, however, may corrupt the data stored in the cell. (because the initial, low voltage corresponded to the “0” stored in the cell, and the higher voltage resulting from the access may cause the data to be ambiguous, or even to flip-flop, so that it is now a “1.”)
If the transistors that make up the components of the memory cell were exactly identical to each other, the probability that the data in the cell would be corrected in this manner would be relatively low. As a practical matter, however, the transistors are not identical, but instead have slight variations that cause them to have slight variations in their respective responses. For example, each transistor has an associated threshold voltage that affects the response of the transistor. Because of manufacturing differences between the transistors, there is a variation in these threshold voltages that, in turn, results in a variation in the transistor's responses. These variations are becoming increasingly significant because the variation in threshold voltages increases as the size of the transistors decreases. Thus, as the memory cells grow smaller, they are more susceptible to the instability problem.
The instability of SRAM cells is obviously problematic. It would clearly be desirable to provide a design for an SRAM cell that is more stable than conventional designs and is therefore less likely to be corrupted when the cell is read.