A body-biasing of readout circuit for STT-RAM
with improved thermal reliability
Lun Yang
, Yuanqing Cheng
y
, Yuhao Wang
z
, Hao Yu
z
, Weisheng Zhao
yx
and Aida Todri-Sanial
{
School of Mathmatics and System Science
y
School of Electrical and Information Engineering Beihang University, Beijing, China 100191
y
School of Electrical and Elec tronic Engineering Nanyang Techn ological University, Singap ore, 639798
z
IEF, U niv. Paris-Sud 1 1, F-91405 Orsay, France
x
LIRMM CNRS/Universit´e d e Montpellier 2, Montpellier,France
Email: yuanqing@ieee.org
Abstract—As the integration density rockets up for contem-
porary VLSI circuits, power consumption li mits the scalability
of technology advancement of CMOS. Spin transfer torque-
magnetic random access memory (STT-MRAM), as one of the
emerging non-CMOS technologies, has the promising prospect of
low standby power, fast access speed and compatibili ty with the
CMOS fabrication process. However, with the technology node s-
caling down, typical 1 Transistor-1 Magnetic Tunnel Junction (1T-
1MTJ) STT-RAM cell suffers from severe reliability challenges,
especially for read op eration under temperature fluctuation. In
this paper, we quant itatively analyze the temperature effect on
read reliability of STT-RAM cell and propose a novel body-
biasing feedback readout circuit design to improve the read
sensing margin under different temperatures. The experiments
based on 40nm CMOS technology and MTJ compact model
validate the eff ectiveness of the proposed method. The improved
sensing margin also permits a smaller sensing current for reading
such that higher read energy efficiency can be achieved.
I. INTRODUCTION
Due to the endless pursuit for high performance computing,
multi-core or many core system comes into play to exploit
parallelism furthe r. However, this design paradigm shifting
also in troduces well-known “dark silicon” effect and elevated
leakage power, especially for memory systems.
Several emerging memory techn ologies, including STT-
MRAM, Phase charge random access memory (PCRAM),
memristor, etc., are proposed to mitiga te the elevating p ower
crisis. Among them, STT-MRAM is o ne of the most promising
candidates to replace SRAM or DRAM thanks to its fast access
speed, extremely low standby power and high integratio n den-
sity. Its promising prospect has already attracted many research
efforts on exploring STT-MRAM pote ntials from device level
to architecture level [1]–[5]. Mo st of the m f ocus on write
energy efficiency and write reliability issues. However, as the
technolo gy node con tinuously scales down, read current has
to be smaller in order to refrain read disturba nces. This makes
the read sensing reliability a severe challenge for STT-RAM
when the feature size shr inks below 50nm [6]. More over, STT-
RAM has to mee t the wide working temperature requirement
ranging from room temperature to 125°C for commercial use.
Therefore, it is also imperative to investigate the tempera ture
impact on STT-RAM access behavior to make sure reliable
data sensing over the whole temperature range. However,
Si/SiO
2
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a
Ta
Ru
Al
2
O
3
Al
2
O
3
CoFeB
CoFeB
MgO
Ta
Ru
Cr/Au
(a)
Free layer
Ba
rrier layer
Reference layer
BL
WL
SL
Bit“0”
Bit“1”
(b)
F
ig. 1: (a) Th e structure of MTJ (b) A typical schematic of
1T-1MTJ STT-RAM cell
few research papers investigate this point, especially from the
circuit design perspective, which motivates the work in this
paper.
In the paper, we first ana lyze megneto-resistive tunnel junc-
tion (MTJ) and tr ansistor behavior for different temperatures.
Then, we evaluate the thermal impact on the typical sen sin g
circuit of 1T-1MTJ cell. Based on the ab ove an alyses, we
propose a novel body-biasing sensing circuit design to improve
the read sensing ma rgin conside ring the temperature variation.
The experimental results show that the sensing reliability can
be enha nced co mpared to the typical read circuit. Moreover,
the sensing energy efficiency can be improved significantly
under the same sensing margin.
The rest of the paper is organized as f ollows. T he prelim-
inaries of STT-RAM ac cess mechanism and related work are
introdu ced in Section II. Section III analyzes the variations of
MTJ and access tra nsistor caused by temperature. After that,
the temperature effect on read sensing margin of a typical
sensing circuit is evaluated in the same section. Based on
above analyses, a novel body-biasing feedback technique is
proposed to improve the r ead sensing ma rgin in Section IV.
Experimental results are shown in Section V. Finally, Section
VI c oncludes the paper.
II. PRELIMINARIES AND RELATED WORK
MTJ is the core of STT-RAM cell. It mainly consists of
an oxide insulating layer (e.g., MgO) sandwiched by two
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