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SRAM
Mohammad Sharifkhani
Effect of Mismatch
Data Retention Voltage
DRV Mote-carlo simulation
Inv. with VL Input @
Read load
Init cond.
VL=1, VH=0
Inv. with VR Input
@ Write load
Shmoo plot
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A shmoo plot is a graph that represents how a particular test passes or fails
when parameters like frequency, voltage, or temperature are varied and the
test is executed repeatedly.
The shape of the failing region is meaningful and helps in determining the
cause of the failure.
A shmoo plot of normal circuit operation shows better high-frequency
performance as supply voltage increases, as shown in Fig. 1a.
Other shapes frequently seen include the curlback (Fig. 1b), ceiling (Fig.
1c), floor (Fig. 1d), wall (Fig. 1e), finger (Fig. 1f), and breaking wave (Fig.
1g).
Techniques for improving reliability
• Read assist circuits
• Write assist circuits
• Error correction methods
Multiple voltages for NM
improvement
Threshold voltage variation
Monte-carlo simulation
Access (reduction
in the BL-differential
produced), Higher Vt 
lower BL swing
Read (data flipping
while reading), lower vtrip
write (unsuccessful
write) Higher Vt 
weaker PU ration
hold (data flipping at a
lower supply voltage in
standby mode)
S. Mukhopadhyay, JSSC 2007
Solution: Adaptive body bias
Dynamic VDD Selection
Higher write noise margin  more power
Read Write-assist circuits
Keeps WL
voltage in
check
(lower for
stable
read)
Charge redistribution
between cell VDD
and down Vdd
Pulsed WL and BL
Minimization of WL activation
Threats write:
Read Modify
Write is used for
all columns
Pulsed WL and BL
On pseudo
read columns
the BLs are
precharged to
a lower voltage
than VDD to
maintain
stability of the
cell
 Weaker
access 
lower delta V to
trip the cell
Dynamic Body Bias
On-chip
programma
ble voltage
generator
with N-well
resistors
Forward BB: stronger PMOS makes a
wider butter fly curve more SNM
during read operation
Higher leakage (only applied on
selected banks)
Read assist
Local SA
Pilo, JSSC’07
Divided BLs
Read assist
When enabled the half
selected BL/read BL get
full swing
Masks the BL
of the half
selected
columns that
do not need full
amplification to
save power
Mask registers are
loaded during
power up
10% more power at
nominal voltage
Yet allows for
1.20.9 VDD
reduction and keeps
the array stable 
saves power at the
end
Write assist
Weaker PMOS is needed
Supply reduction of the to be written cells by ~200mV
Only the columns to be
written on get the lower
supply voltage: a power
decoder is needed
Reference generator
Bi-directional/data
dependent current
flow
Write voltage
Old data: VDDVWR
New data: sink data from VWR to charge up the new 1 node
Writing old data pulls up VWR  push-pull is needed at Ref. Generator
Redundancy in SRAMs
Redundancy
Error Correction Code
Multi-bit errors
Multi-bit errors: Interleaving
Future trends
• More than 6T cells
• Change in technology
• eDRAM
More transistors
Thin Body MOSFETs
Double Gate FinFET
Double Gate vs. Tri-Gate
Independent Gate operation
Applications
Independent Gate Operation
6-T SRAM in Bulk-Si
6T SRAM with FinFET
6T SRAM with 2 FinFET
Embedded DRAM