A High-Density Subthreshold SRAM

with Data-Independent Bitline Leakage

and Virtual Ground Replica Scheme

Tae-Hyoung Kim, Jason Liu,

John Keane, and Chris H. Kim

University of Minnesota, Minneapolis

2/23

Agenda

• Introduction to Subthreshold Operation

• Proposed Subthreshold SRAM– Decoupled 10T SRAM Cell

– Write margin improvement utilizing Reverse Short Channel Effect (RSCE)

– Data-independent bitline leakage

– Virtual ground replica scheme

– Write-back scheme for stability

• 0.2V 480kb SRAM Measurements

• Conclusions

3/23

Introduction to Subthreshold Operation

• Characteristics– VGS is smaller than VTH

– Exponential current equation

– For low power, minimum energy applications

• Applications of Subthreshold SRAM– Wireless sensor nodes: data memory for DSPs

– Medical devices: hearing aids, pacemakers

– Portable electronics: cellular phones, PDAs

)1(0

t

DS

t

thGS

V

V

mV

VV

eff

DD eeL

WII

−−−−−−−−

−−−−====

4/23

Subthreshold SRAM Design Issues

• Mainly due to small Ion-to-Ioff ratio and current variation in subthreshold

– Read failure

: Reduced SNM, small bitline sensing margin

– Write failure

: Weak write path

– Limited array efficiency

: Data-dependent bitline leakage

– Performance and power variation

5/23

SNM of 6T SRAM Cell

QB (m

V)

RB

LB

RBL

• Read SNM is 18% of hold SNM at 0.2V• Read SNM is too small for robust subthreshold

operation

0

1

1

1

0

1

1

1

6/23

SNM of Proposed 10T SRAM Cell

0

1

1

1

• Decoupled cell node

• Read SNM is equal to hold SNM

• SNM = 38% of supply voltage

• Stability only limited by cross-coupled latch

VD

D

7/23

Write Margin Improvement

• Conventional Techniques

– Sizing: access TR vs. PMOS in latch

– Higher WL voltage for access TR

– Virtual VDD

• Proposed

– Utilize Reverse Short Channel Effect (RSCE) to strengthen write path

RBLB

RB

L

Higher voltage

Sizing

8/23

HALO Impact in Subthreshold

pppp++++

VDDVDDVDDVDD====1111....2222VVVV

pppp++++nnnn++++ nnnn++++

pppp++++ pppp++++nnnn++++ nnnn++++

HaloHaloHaloHalo

VDDVDDVDDVDD====0000....2222VVVV

Strong Strong Strong Strong

DIBLDIBLDIBLDIBL

WeakWeakWeakWeak

DIBLDIBLDIBLDIBL

SS SSuu uurr rr ff ff

aa aacc ccee ee dd dd

oo oopp ppii ii nn nn

gg gg

aa aacc ccrr rr oo oo

ss ssss ss cc cc

hh hhaa aann nnnn nnee eell ll High dopingHigh dopingHigh dopingHigh doping

Depletion Depletion Depletion Depletion

regionregionregionregion • HALO to mitigate Short Channel Effect (SCE) in superthreshold

• Negligible SCE in subthreshold region

• Dominant RSCE in subthreshold region

9/23

Utilizing RSCE for ImprovedDrive Current

• Optimal channel length in subthreshold region– Lopt=0.55µm for max. current/width– Lopt=0.36µm for max. performance/width

Channel Length (µm)

0

01

3

1

2

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1

2

T. Kim, et al., ISLPED06

Lmin =0.13μμμμm

10/23

Utilizing RSCE for Write Margin Improvement

• Write access TRs utilizing RSCE (L=3xLmin)

• Increased current drivability: 3.3x at 0.2V

• Write margin improvement of 70mV at 0.2V

TR utilizing RSCEAdditional 8.5% area increase

11/23

Utilizing RSCE for Delay Improvement

• Narrower width for the same current drivability

• Reduced junction capacitance

• Improved delay due to reduced capacitance

Fat NMOS devices to utilize RSCE

12/23

Data-Dependent BL Leakage Problem

• Undetermined region due to data-dependency

• Possible read failure using single threshold read buffer

RBL

RB

L

ISSCC06

13/23

Proposed Scheme with Data-Independent BL Leakage

• Bitline logic high and low levels are fixed

• Logic level difference of 130mV at 0.2V with 1kcells/bitline

Q

N-1 Cells

RWL

RWLQ

RWL

Accessed Cell

N-1 Cells

Q= High Q= Low

Accessed Cell

RWL

RWL

RWL

QBB

14/23

Virtual Ground Replica Scheme

• Conventional: fixed switching threshold

• This work: VGND tracks RBL logic low level

15/23

Virtual Ground Replica Scheme

• Shared VGND across multiple read buffers• Replica bitline with fixed data to generate VGND

VG

ND

16/23

Pseudo-Write Problem

• Pseudo-write problem in unselected columns

• Worst case SNM due to current path

0 1 1 1

17/23

Write-Back Technique

• Write after read to solve pseudo-write problem

• Write-back to unselected columns

18/23

480kb SRAM Chip Implementation

19/23

Vmin and Leakage Measurements

0.14

0.15

0.16

0.17

0.18

1 2 3 4

1

10

100

0 0.2 0.4 0.6 0.8 1 1.2

10.2µA @ 0.2V, 27°C

(480kb SRAM)

0.17V @ 1k cells/

bitline, 27°C

• Vmin_read = 0.17V @ 1k cells/bitline

• Vmin_write = 0.20V

• 90% Ileak reduction at 0.2V compared to 1.2V

20/23

Performance Measurements

• 4.5x delay difference between four quadrants

• Exponential delay reduction with higher supply

0

2

4

6

8

10

12

14

0.1 0.2 0.3 0.4 0.5 0.6

0

2

4

6

8

10

12

128 256 512 1024

21/23

VGND Measurements

• Exponential increase of normalized VGND

• 0.1V bitline swing at VDD=0.2V

• VGND relatively constant with temperature

RBL

22/23

Improved Delay Utilizing RSCE

• Differential delay measurement circuit

• 28% delay improvement in predecoderutilizing RSCE

Td1+Td2Td1+Td_ckt+Td2

P: Proposed, C: Conventional

23/23

Conclusions

• Circuit techniques proposed to enable a 0.2V 480kb subthreshold SRAM

– Decoupled 10T cell for improved read stability

– Utilization of RSCE for improved write margin

– Data-independent leakage for enabling 1k cells/bitline

– Virtual ground replica scheme for optimal read buffer sensing margin

– Write-back scheme to eliminate pseudo-write problem