A 45-nm Bulk CMOS Embedded SRAM With Improved Immunity Against Process and Temperature Variations

Koji Nii, Makoto Yabuuchi, Yasumasa Tsukamoto, Shigeki Ohbayashi, Susumu Imaoka, Hiroshi Makino, Yoshinobu Yamagami, Satoshi Ishikura, Toshio Terano, Toshiyuki Oashi, Keiji Hashimoto, Akio Sebe, Gen Okazaki, Katsuji Satomi, Hironori Akamatsu, Hirofumi Shinohara

Research output: Contribution to journalArticlepeer-review

52 Citations (Scopus)


The variation tolerant assist circuits of an SRAM against process and temperature are proposed. Passive resistances are introduced to the read assist circuit with replica memory transistors to lower the wordline voltage accurately reflecting the process and temperature variations. For the sake of not only enlarging the write margin but also reducing power consumption and speed overhead, the divided dynamic power-line scheme based on a charge sharing is adopted. Test chips of 512-Kb SRAM macros and isolated memory cell TEGs are fabricated using 45-nm bulk CMOS technology. Two types of 6-T SRAM cells, whose sizes were 0.245 m2 and 0.327 m2 were designed and evaluated. From the measurement results, we achieved over 100-mV improvement for static noise margin, and 35 mV for write margin for both SRAM cells at 1.0-V worst condition by using assist circuitry. It enables the wordline level to keep higher voltage at the slowest condition than the typical process condition, which results in 83% improvement of the cell current compared with the conventional assist circuit. Furthermore, the minimum operating voltage in the worst case condition was improved by 170 mV, confirming a high immunity against process and temperature variations with less than 10% area overhead.

Original languageEnglish
Pages (from-to)180-191
Number of pages12
JournalIEEE Journal of Solid-State Circuits
Issue number1
Publication statusPublished - 2008 Jan
Externally publishedYes


  • 45-nm bulk CMOS
  • Assist circuit
  • SRAM
  • V variation
  • memory cell
  • read margin
  • static noise margin (SNM)
  • write margin

ASJC Scopus subject areas

  • Electrical and Electronic Engineering


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