Memory Designing Using
Josephson Gates
Susmit Biswas
02/07/2006
Outline
Refreshing Memory
Memory Circuits
CMOS Memory Circuits
Need For New Memory Technology
Josephson PC Memory
Previous Work
Josephson Junction
Memory Designing Using Josephson Gate
Performance Evaluation
Conclusion
Standard Memory Technology
The Memory Hierarchy
CPU Registers
L1 Cache (SRAM)
L2 Cache (SRAM)
Main Memory
SRAM
DRAM
FPM DRAM (Fast Page Mode DRAM)
EDORAM (Extended Data Out DRAM )
SDRAM (Synchronous DRAM)
DDR DRAM (Double Data Rate DRAM)
DRAM
High Density and low power
but Slower than SRAM
DRAM Performance
(August 2005)
Need For New Technology
Memory is the main bottleneck now
Multiprocessor system suffers most
SIMD and MIMD architecture
Data hungry
Josephson Memory: Previous Work
Josephson Junction:
Discovered and Demonstrated in early 60’s
IBM till 1983
Nearly functional 1kBit memory using lead
-
alloy
1980s : ETL, NTT using Nb/Al0x/Nb
1993 : UC Berkeley designed a 4 kBit RAM
1997 : NEC developed a 4 kBit Memory
2002 : Hybrid Josephson memory
Looking Back
1962
:
Josephson
predicted
that
a
sandwich
of
S
-
I
-
S
will
show
remarkable
properties
when
the
insulator
is
sufficiently
thin
~
10
Å
or
so
Current
can
flow
through
the
junction
with
no
voltage
appearing
across
the
junction
until
a
critical
current
I
J
is
exceeded
The
magnitude
of
I
J
,
depends
sensitively
on
magnetic
fields
.
A
voltage
V
dc
,
impressed
across
the
junction
leads
to
an
oscillating
supercurrent
whose
frequency
is
proportional
to
the
voltage
.
The
frequency
is
very
high
for
even
modest
voltages
(
483
MHz/
μ
V)
.
Josephson Effect
Two
-
fluid
model
of
Superconductor
:
One
of
the
fluids
is
the
normal
fluid,
the
other
the
superfluid
.
Superfluid
consists
of
paired
electrons
(Cooper
pairs)
of
equal
but
opposite
momentum
and
spin
Josephson Effect
Bound
pairs
electrons
all
lie
near
the
Fermi
energy
E
F
of
the
normal
metal
;
the
resulting
pairs
are
in
an
energy
state
lower
than
E
F
by
an
amount
Δ
(
binding
energy
of
the
pair
(per
electron)
As T becomes less than T
c
, pairs begin to
form and condense into the superconducting
state
At
V
= 2
Δ
/e
the tunneling current increases
sharply (with +
∞
slope)
For
V >> 2
Δ
/e
the current increases linearly
with
V
Josephson Junction
Josephson
Effect
:
In
superconducting
state
of
certain
metals,
electrons
are
attracted
by
each
other
and
form
bound
pairs,
called
Cooper
pairs
.
When
these
pairs
of
electrons
tunnel
through
a
thin
insulating
barrier
placed
between
two
superconductors,
the
whole
is
called
Josephson
junction
.
Josephson Junction Characteristics
Control currents
I
c
,
Josephson threshold
I
m
.
Gate current
I
g
,
I
-
V Curve
Threshold Curve
Josephson Junction As Memory
Consists of a loop with three Josephson junctions in
series that encloses a magnetic flux
Ф
driven by an
external magnet.
The loop may have multiple stable persistent current
states when the enclosed magnetic flux is close to half a
superconducting flux quantum
Ф
Ф
=
h
/
2
e
System has two stable states
׀
0
›
and
׀
1
›
with opposite
circulating persistent currents
Josephson Junction As Memory (
cont
.)
Operated
by
resonant
microwave
modulation
of
the
enclosed
magnetic
flux
by
a
superconducting
control
line
on
top
of
the
qubit,
separated
by
a
thin
insulator
.
The
state
of
a
bit
(
0
or
1
)
depends
on
the
sum
of
the
external
magnetic
flux
generated
by
the
circulating
currents
on
the
surrounded
loops
:
0 if magnetic field is < 1/2
Ф
1 if magnetic field is >
1/2
Ф
The
state
of
the
system
is
the
superposition
of
all
the
states
generated
by
the
circulating
current
in
each
loop
.
Josephson Junction As Memory (
cont
.)
Combining several junctions results in different gates
e.g. inverter
Can be designed in two ways
coupling two superconductive loops directly through
magnetic interference
Coupling two loops through a superconductive flux
transporter
Josephson Junction As Memory (
cont
.)
Stronger interaction between the PC loops and better coupling to
each other with the facilitation of transporter
But!
Coupling between neighboring loops makes it difficult for long
-
range
communication
Solution
Transporter: fast data propagation
Josephson Junction As Memory (
cont
.)
NMV Gate can serve as NAND, NOR and NOT gate by setting
instruction bits.
Not Majority Vote (NMV) Gate
Memory Designing Using
Josephson Gate
Memory Designing Using
Josephson Gate (cont.)
A memory cell can not be refreshed by either a row or a
column addressing line independently
the
addressing
lines
are
designed
in
such
a
way
that
the
states
of
other
cells
in
the
same
column
are
suppressed
during
reading,
the
selected
one
gets
the
bit
from
its
adjacent
memory
cell,
without
interacting
with
its
neighbors
in
the
same
column
.
Performance Evaluation
Pros:
Speed
:
750GHz
for single asynchronous cells and up to
320GHz
for LSI
devices
Low power
consumption
0.2nanowatt/GHz
per pulse and
0.1mW
for LSI devices
Simple fabrication technology : lithography
Cons:
Low density
Operational temperature <20K
Performance Evaluation (cont.)
Comparison
of
projected
2
.
5
μ
m
technology
Josephson
NDRO
and
DRO
chip
designs
with
advanced
silicon
memories
having
comparable
line
widths
.
Conclusion
Josephson memory can become more and more
popular because of its speed and low power
characteristics
Designing larger memory is difficult
Low density
Limitation of fabrication technology
References
1.
“Novel Computing Architecture on Arrays of
Josephson Persistent Current Bits”
:
Jie Han, Pieter
Jonker [
Proc. MSM
2002
]
2.
“Memory
-
Cell Design in Josephson Technology” :
Hans H. Zmpe [IEEE Transactions On Electron
Devices, VOL. ED
-
27
, NO.
10
, OCTOBER
1980
]
3.
“
570
-
ps
13
-
mW Josephson
1
kbit NDRO RAM” :
Shuichi Nagasawa
et. al.
[IEEE Journal of Solid
-
State
Circuits, Vol
24
, No
5
, October,
1989
]
References
4.
“
Design Of A 16
-
kbit Variable Threshold Josephson
RAM
”: I. Kurosawa, [IEEE Transactions On Applied
Superconductivity, Vol. 3, No.l, March1993]
5.
“Josephson Type Superconductive Tunnel Junctions
and Applications”
: Juri Matisoo [
IEEE
TRANSACTIONS ON XAGNETICS, DECEMBER
1969]
6.
http
://www.lne.fr/en/r_and_d/electrical_metrology/josep
hson_effect_ej.shtml
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