Ultra-Low-Power SRAM Design In High
Variability Advanced CMOS
AUG 07 2009
Submitted to the Department of Electrical Engineering and Computer
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
@ Massachusetts Institute of Technology 2009. All rights reserved.
Department of Electric
ngineering and Computer Science
May 5, 2009
C ertified by ...............
Anantha P. Chandrakasan
Joseph F. and Nancy P. Keithley Professor of Electrical Engineering
Accepted by ..........................
Terry P. Orlando
Chairman, Department Committee on Graduate Theses
Ultra-Low-Power SRAM Design In High Variability
Submitted to the Department of Electrical Engineering and Computer Science
on May 5, 2009, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
Embedded SRAMs are a critical component in modern digital systems, and their role
is preferentially increasing. As a result, SRAMs strongly impact the overall power,
performance, and area, and, in order to manage these severely constrained trade-offs,
they must be specially designed for target applications. Highly energy-constrained
systems (e.g. implantable biomedical devices, multimedia handsets, etc.) are an
important class of applications driving ultra-low-power SRAMs.
This thesis analyzes the energy of an SRAM sub-array. Since supply- and threshold-
voltage have a strong effect, targets for these are established in order to optimize
energy. Despite the heavy emphasis on leakage-energy, analysis of a high-density
256x256 sub-array in 45nm LP CMOS points to two necessary optimizations: (1) ag-
gressive supply-voltage reduction (in addition to Vt elevation), and (2) performance
enhancement. Important SRAM metrics, including read/write/hold-margin and read-
current, are also investigated to identify trade-offs of these optimizations.
Based on the need to lower supply-voltage, a 0.35V 256kb SRAM is demonstrated
in 65nm LP CMOS. It uses an 8T bit-cell with peripheral circuit-assists to improve
write-margin and bit-line leakage. Additionally, redundancy, to manage the increas-
ing impact of variability in the periphery, is proposed to improve the area-offset
trade-off of sense-amplifiers, demonstrating promise for highly advanced technology
nodes. Based on the need to improve performance, which is limited by density con-
straints, a 64kb SRAM, using an offset-compensating sense-amplifier, is demonstrated
in 45nm LP CMOS with high-density 0.25 pm2 bit-cells. The sense-amplifier is re-
generative, but non-strobed, overcoming timing uncertainties limiting performance,
and it is single-ended, for compatibility with 8T cells. Compared to a conventional
strobed sense-amplifier, it achieves 34% improvement in worst-case access-time and
4x improvement in the standard deviation of the access-time.
Thesis Supervisor: Anantha P. Chandrakasan
Title: Joseph F. and Nancy P. Keithley Professor of Electrical Engineering
MIT is truly a unique and wonderful place on this earth. For a new graduate student,
as I once was, it can easily be too wonderful and too big. The only way to realize your
place at MIT is through the guidance, encouragement, support, and friendship of an
outstanding advisor like Prof. Anantha Chandrakasan. First and foremost, I thank
Anantha. When I arrived here, I was not sure what, if anything, I could accomplish.
Anantha, convinced me, by always expecting more from me, by always challenging me,
and by supporting me through every research endeavor, that I could be a contributing
member of this great community. His lessons for me have gone far beyond circuits;
he has taught me to be a critical, sincere, cooperative, and respectful researcher.
Anantha works firstly for his students, and I have learned more by watching him than
I ever will from reading volumes of journals. As I proceed in my career, Anantha will
always play an important role; he has given me something to strive for technically
and personally. Thank you, Anantha, for your always strong support and guidance.
I am eternally grateful to my thesis committee members, Prof. Charlie Sodini
and Prof. Duane Boning. Every researcher offers his work to the community hoping
it is received by someone. To be able to discuss my work with such outstanding
researchers as Charlie and Duane is the greatest honor of my career. Charlie and
Duane have given this thesis a level of attention that has made the effort more than
worthwhile. Thank you for your feedback and support, which has always aimed to
make this thesis better. Because of your input, I am much prouder of this work, and
after the many years it has consumed, that means a lot!
There are several faculty at MIT who have had a profound impact on me both
technically and non-technically. I am extremely grateful to Prof. Harry Lee, who's
mastery of circuits, and the ability to make that mastery accessible, has inspired me
to study every last aspect of my field. I am grateful to Prof. Al Oppenheim who,
by example, has shown me the impact that excellence in teaching can have and the
level of dedication that must applied. I thank Prof. John Guttag for encouraging me
to enthusiastically and intrepidly venture into new fields to seek out for myself how I
might broaden my contributions. Finally, I thank Prof. Joel Dawson for showing me
that a newbie can have as big an impact as anyone, and he can do so without strain
or tension, smiling all the way.
By far the most rewarding aspect of MIT has been the people I have been so
fortunate to interact with. First, I must thank Margaret, who has repeatedly rescued
me from overloads and crises. Margaret keeps ananthagroup running straight even
when us students have accidentally gone in the wrong direction! Technically, the most
fun I have ever had was discussing, debating, and pondering with Brian Ginsburg
on matters of how to design an ADC (yes, many of the problems we hotly contested
were already solved, but sometimes re-inventing the wheel is an unmatchable learning
exercise!). I will always remember those years spent with Brian twisting my brain
in front a white-board. Past members of ananthagroup, especially Benton Calhoun
and David Wentzloff, showed me the ropes of being a graduate student. This, as
they taught me, involves more than just tape-outs and paper deadlines; it involves
lunch-time business plans, political/social debates, "useless" riddles and anecdotes,
and most of all, laughs wherever they can be found. Also in this category are Alice
Wang, Frank Honore, Fred Lee, and Raul Blazquez.
I am privileged to have the current members of ananthagroup around me every-
day. I am especially grateful for the technical discussions and collaborations of Joyce
Kwong, Yogesh Ramadass, and Nigel Drego (I will have more to say about these last
two clowns shortly). I must thank my good friend Manish Bhardwaj, not just for
his technical feedback but also for his support and encouragement, which was always
on-hand when I needed it most (like when he put in a late night of chip testing with
me to get results that were due the previous week!). Daniel Finchelstein, Denis Daly,
and I arrived at MIT together, and I have had these two to lean on throughout my
time here. They are the best fellow travellers one can hope for on this sort of journey,
and I am grateful for their friendship the whole way through. It is also inspiring to
see the newer students in the group, Vivienne Sze, Mahmut Ersin Sinangil, Patrick
Mercier, and Masood Qazi, excelling and indeed becoming leaders.
I have especially been looking forward to say something about my friend Ali Shoeb.
His hyperactivity and enthusiasm are the main reasons why I will continually seek to
expand and broaden my horizons beyond any narrow expertise I might have. Ali is
genuinely inspired, and he inspires me! Eugene Shih is more controlled, but he has
contributed equally to the fun I have had on the ninth flour of Building 32!
Thankfully, my experiences at MIT have actually gone far beyond MIT. I am
extremely grateful for the support and encouragement I have received from collabo-
rators at Texas Instruments. Most of all, Dennis Buss has been a champion of my
work throughout my Ph.D. years. His enthusiasm has been a constant driving force,
and he has spun miracles for me on more than one occasion to overcome the barriers
and hurdles that inevitably arise during research. I am also grateful to Ted Houston,
Wah-Kit Loh, Xiaowei Deng, Mike Clinton, Hugh Mair, and Alice Wang for their
constant support and feedback.
I am thankful to Intel for providing me with fellowship support during my Ph.D.
Even more importantly, Kevin Zhang of Intel has played a major role in how I have
approached SRAMs from the research perspective. In fact, much of the work in this
thesis has been inspired by his own research and the feedback he has been so generous
to me with. Kevin has been a constant supporter and a mentor who I will always
look to for stimulating discussions and input.
I am also thankful to Peter Holloway of National Semiconductor. It is much easier
to do research when one has the kind of support that Peter has given me throughout
my Ph.D. Peter has a unique perspective on circuits that is rooted in real-life; the
only way a novice like myself can appreciate such a perspective is through the very
intriguing and stimulating discussions I have had with him.
Completing a Ph.D. is far more than a test of technical execution. In fact, most
of all, it is a test of will and morale. For both of these I am eternally grateful to the
close friends I have made during my time here at MIT. Some of my most important
moments at MIT have been spent during coffee-time with Nigel Drego and Yogesh
Ramadass. Here, we got to transfer our analysis skill to all of life's great problems.
None of us knows if we ever came close or even began to solve any of these, but
we always returned from coffee less stressed, more motivated, and of course slightly
more awake... any way you cut it coffee-time is indispensible! Yogesh, Nigel, Vidya,
Anand, and Nammi are great friends, and we are truly blessed to be able to laugh,
lounge, and talk smack with them. The same, of course, goes for Daniel and Tarik
(and Minou!). Since I arrived here at MIT Raj, Ferdi, Federico, and Gabi have been
the rough-around-the-edges group with whom I could always be myself. This turns
out to be a critical outlet when the pressure begins mounting, as it frequently does
Finally, I come to my family, without whom nothing in my life, let alone my
research, could ever have been possible. Most of all, my hard work and sincere efforts
are for Mom Ji and Dad Ji. I have always relied on your love and prayers to lift me
over obstacles. Of course, Vancouver is a continent away, but I have always felt you
here with me, and that has been the strength I have needed. This thesis is for both
of you. Thank you for your support, love, and blessings.
So far as effort put into this thesis is concerned, the first credit undoubtedly goes
my amazing wife Anita. Ana, you are the reason behind this accomplishment, and
your smile (and occasional craziness!) are the only rewards I hope for every day.
Thank you for your love and support. I love you with all my heart.
I am blessed to also have the support and love of a second set of parents. Mom
and Bug, thank you for your prayers, wishes, jokes, and love. I do not expect you to
read this thesis, but I do hope you realize the role you have played in supporting me
towards its completion. Thank you, once again, for your support, love, and blessings.
I am anxious to thank Angelee, Serena, and Jaimini. You three remind me that
there is a lot more to my life than whatever I am busy with today. Thank you for the
relief and lightening that your support and love always provides. This thesis truly
could not have been completed without the formidable force behind me that you three
have always been.
Similarly, Ang, Jason, and Connor, I know that you are always behind me and
Ana, and we are externally grateful for the love, laughs, and lessons (about leather-
backed turtles, etc.) that you have always provided.
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