Dram storage capacitor without a fixed voltage reference

Abstract

In one embodiment, a DRAM is provided that includes a plurality of memory cells, each memory cell including an access transistor and a storage capacitor, wherein the storage capacitor includes a first node coupled to the access transistor and a second node isolated from the first node, the second node comprising signal-bearing metal conductors.

Description

RELATED APPLICATION[0001]This application claims the benefit of U.S. Provisional Application No. 60 / 913,542, filed Apr. 24, 2007 and U.S. Provisional Application No. 60 / 941,667, filed Jun. 3, 2007.BACKGROUND OF THE INVENTION[0002]The present invention relates to memories, and more particularly to a dynamic random access memory (DRAM) architecture.[0003]Because a DRAM memory cell includes just a single access transistor and a storage capacitor, DRAM offers dramatic density advantages over static random access memory (SRAM), which typically requires a six transistor (6-T) memory cell. In contrast to SRAM, a DRAM cell will only change the voltage on its corresponding bit line slightly during read operations. Having been coupled to the bit line, the storage capacitor in a DRAM cell must be then be restored after a read operation. Thus, DRAM sense amplifiers require a regenerative latching ability to drive the bit line “full rail” after a read operation. If the sense amplifier determines...

AI Technical Summary

Problems solved by technology

Moreover, the charge on DRAM storage capacitors continually leaks away, requiring constant refreshing.

As a result of the density vs. speed advantages of SRAM and DRAM, SRAM is faster but more expensive and thus reserved for more time-critical operations such as a microprocessor's cache.

As a result, the maximum number of memory cell rows per sense amplifier in a conventional trench-capacitor DRAM is limited to, for example, 512 rows per sense amplifier.

Thus, embedded DRAMs are particularly sensitive to bit-line-capacitance-density-limiting-issues.

Another hindrance to increasing density is the capacitance of the access transistor.

However, each access transistor is not a perfect switch for such coupling as it has an inherent capacitance between its gate and its drain / source terminals.

Another factor in increasing density is the non-ideal characteristic of sense amplifiers.

Should both differential inputs, however, be at the same voltage (such as a pre-charge voltage VDD / 2), the regenerative latch operation just described may not take place correctly.

In turn, this offset limits the sensitivity of the sense amplifier operation.

If the storage capacitance is too small with regard to other effects such as bit line capacitance, the offset within the differential amplifier may drive the regenerative latch to pull output N full rail and ground output P, leading to an erroneous reading.

However, the increased storage capacitance generally leads to increased memory cell size, thereby diminishing density.

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