A Comprehensive Review of Design Challenges and Techniques for Nanoscale SRAM: A Cell Perspective

Abstract

<p>In order to meet the ultra-low power requirement of modern digital systems, voltage scaling<br> is a fruitful technique that is widely adopted. However, the voltage scaling at ultra-scaled technologies<br> significantly affect the stability of an Static Random Access Memory (SRAM) cell. This work provides<br> an extensive review of various designs and techniques for solving issues related to SRAM cell stability.<br> Decoupling the read operation is a commonly used technique for addressing read stability issue. Modified<br> Schmitt-trigger based cell core, loop-cutting and additional discharge path based techniques have also been<br> employed in the literature to enhance the read stability. However, the write-ability and stability of halfselected (HS) cells have been compromised in some of these technique. For write-ability enhancement,<br> most of the techniques involve either weakening of pull-up/pull-down devices (to reduce the current<br> responsible for storing the data) or strengthening of access transistors (to enhance the write current). It<br> includes VDD/VSS -cut off or -floating, feedback cutting, multi-Vth write assist based cells. Apart from<br> discussing the alternative bitcells, we also cover several peripheral read/write assist techniques. WL underdrive (WLUD) and dynamic WLUD are effective read assist techniques. WL -boost, WL -delayed boost,<br> VDD-floating/lowering, transient VDD collapse, VSS raised, dual split control-VSS raised and negative BL<br> (NBL) are among the peripheral techniques for enhancing write-ability. Furthermore, various techniques<br> for mitigating the HS cell issues, such as write-back, column decoupled local wordline, modified bitinterleaving array architecture are discussed. Finally, the Cross-point selection based cell accessing in-built<br> technique is illustrated. Through summary tables, conclusions are also derived for suggesting a circuit of<br> choice for various categories. <br> </p>