Memory Designing Using

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