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A Novel Column-Decoupled 8T Cell for Low-Power Differential and Domino-Based SRAM Design

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3 Author(s)
Rajiv V. Joshi ; T.J. Watson Research Labs, IBM, Yorktown Heights ; Rouwaida Kanj ; Vinod Ramadurai

We present a novel half-select disturb free transistor SRAM cell. The cell is 6T based and utilizes decoupling logic. It employs gated inverter SRAM cells to decouple the column select read disturb scenario in half-selected columns which is one of the impediments to lowering cell voltage. Furthermore, “false read” before write operation, common to conventional 6T designs due to bit-select and wordline timing mismatch, is eliminated using this design. Two design styles are studied to account for the emerging needs of technology scaling as designs migrate from 90 to 65 nm PD/SOI technology nodes. Namely we focus on a 90 nm PD/SOI sense Amp based and 65 nm PD/SOI domino read based designs. For the sense Amp based design, read disturbs to the fully-selected cell can be further minimized by relying on a read-assist array architecture which enables discharging the bit-line (BL) capacitance to GND during a read operation. This together with the elimination of half-select disturbs enhance the overall array low voltage operability and hence reduce power consumption by 20%-30%. The domino read based SRAM design also exploits the proposed cell to enhance cell stability while reducing the overall power consumption more than 30% by relying on a dynamic dual supply technique in combination of cell design and peripheral circuitry. Because half-selected columns/cells are inherently protected by the proposed scheme, the dynamic supply “High” voltage is only applied to read selected columns/cells, while dynamic supply “Low” is employed in all other situations, thereby reducing the overall design power. A short bitline loading of 16 cells/BL is adopted to achieve high-performance low-power operation and lower bitline capacitance to improve stability. A newly developed fast Monte Carlo based statistical method is used to analyze such a unique cell, and 65 nm design simulations are carried out at 5 GHz. The feasibility of the cell and sensitivi- y to sense Amp timing has been proved by fabricating a 32 kb array in a 90-nm PD/SOI technology. Hardware experiments and simulation results show improvements of cell Vddmin over traditional 6T cells by more than 150 mV for 90 nm PD/SOI technology. Also experimental results based on fabricated 65 nm PD/SOI (1.6 kb/site × 80 sites) hardware also asserts half-select disturb elimination and hence the ability to enable significant power savings. The performance and speed are shown to be comparable with the conventional 6T design.

Published in:

IEEE Transactions on Very Large Scale Integration (VLSI) Systems  (Volume:19 ,  Issue: 5 )