<s>
Phase-change	B-Device
memory	I-Device
(	O
also	O
known	O
as	O
PCM	O
,	O
PCME	O
,	O
PRAM	O
,	O
PCRAM	B-Device
,	O
OUM	O
(	O
ovonic	B-Device
unified	I-Device
memory	I-Device
)	O
and	O
C-RAM	O
or	O
CRAM	O
(	O
chalcogenide	B-Device
RAM	I-Device
)	O
)	O
is	O
a	O
type	O
of	O
non-volatile	B-General_Concept
random-access	I-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
PCM	O
also	O
has	O
the	O
ability	O
to	O
achieve	O
a	O
number	O
of	O
distinct	O
intermediary	O
states	O
,	O
thereby	O
having	O
the	O
ability	O
to	O
hold	O
multiple	O
bits	O
in	O
a	O
single	O
cell	O
,	O
but	O
the	O
difficulties	O
in	O
programming	O
cells	O
in	O
this	O
way	O
has	O
prevented	O
these	O
capabilities	O
from	O
being	O
implemented	O
in	O
other	O
technologies	O
(	O
most	O
notably	O
flash	B-Device
memory	I-Device
)	O
with	O
the	O
same	O
capability	O
.	O
</s>
<s>
Other	O
research	O
has	O
focused	O
on	O
the	O
development	O
of	O
a	O
GeTe	O
–	O
Sb2Te3	O
superlattice	B-General_Concept
to	O
achieve	O
non-thermal	O
phase	O
changes	O
by	O
changing	O
the	O
co-ordination	O
state	O
of	O
the	O
germanium	O
atoms	O
with	O
a	O
laser	O
pulse	O
.	O
</s>
<s>
This	O
new	O
Interfacial	O
Phase-Change	B-Device
Memory	I-Device
(	O
IPCM	O
)	O
has	O
had	O
many	O
successes	O
and	O
continues	O
to	O
be	O
the	O
site	O
of	O
much	O
active	O
research	O
.	O
</s>
<s>
Leon	O
Chua	O
has	O
argued	O
that	O
all	O
two-terminal	O
non-volatile-memory	B-General_Concept
devices	O
,	O
including	O
PCM	O
,	O
should	O
be	O
considered	O
memristors	O
.	O
</s>
<s>
More	O
recently	O
,	O
interest	O
and	O
research	O
have	O
resumed	O
as	O
flash	B-Device
and	O
DRAM	O
memory	O
technologies	O
are	O
expected	O
to	O
encounter	O
scaling	O
difficulties	O
as	O
chip	O
lithography	O
shrinks	O
.	O
</s>
<s>
Chalcogenide	O
is	O
the	O
same	O
material	O
used	O
in	O
re-writable	O
optical	B-Device
media	I-Device
(	O
such	O
as	O
CD-RW	O
and	O
DVD-RW	O
)	O
.	O
</s>
<s>
However	O
,	O
a	O
January	O
2006	O
Samsung	B-Application
Electronics	O
patent	O
application	O
indicates	O
PRAM	O
may	O
achieve	O
switching	O
times	O
as	O
fast	O
as	O
five	O
nanoseconds	O
.	O
</s>
<s>
Each	O
of	O
these	O
states	O
has	O
different	O
electrical	O
properties	O
that	O
can	O
be	O
measured	O
during	O
reads	O
,	O
allowing	O
a	O
single	O
cell	O
to	O
represent	O
two	O
bits	O
,	O
doubling	O
memory	B-Device
density	I-Device
.	O
</s>
<s>
Phase-change	B-Device
memory	I-Device
devices	O
based	O
on	O
germanium	O
,	O
antimony	O
and	O
tellurium	O
present	O
manufacturing	O
challenges	O
,	O
since	O
etching	O
and	O
polishing	O
of	O
the	O
material	O
with	O
chalcogens	O
can	O
change	O
the	O
material	O
's	O
composition	O
.	O
</s>
<s>
PRAM	O
's	O
switching	O
time	O
and	O
inherent	O
scalability	O
make	O
it	O
more	O
appealing	O
than	O
flash	B-Device
memory	I-Device
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
works	O
by	O
modulating	O
charge	O
(	O
electrons	O
)	O
stored	O
within	O
the	O
gate	O
of	O
a	O
MOS	B-Architecture
transistor	I-Architecture
.	O
</s>
<s>
The	O
gate	O
is	O
constructed	O
with	O
a	O
special	O
"	O
stack	O
"	O
designed	O
to	O
trap	O
charges	O
(	O
either	O
on	O
a	O
floating	O
gate	O
or	O
in	O
insulator	B-Algorithm
"	I-Algorithm
traps	I-Algorithm
"	I-Algorithm
)	O
.	O
</s>
<s>
The	O
presence	O
of	O
charge	O
within	O
the	O
gate	O
shifts	O
the	O
transistor	B-Application
's	O
threshold	O
voltage	O
higher	O
or	O
lower	O
,	O
corresponding	O
to	O
a	O
change	O
in	O
the	O
cell	O
's	O
bit	O
state	O
from	O
1	O
to	O
0	O
or	O
0	O
to	O
1	O
.	O
</s>
<s>
General	O
write	O
times	O
for	O
common	O
flash	B-Device
devices	O
are	O
on	O
the	O
order	O
of	O
100	O
μs	O
(	O
for	O
a	O
block	O
of	O
data	O
)	O
,	O
about	O
10,000	O
times	O
the	O
typical	O
10	O
ns	O
read	O
time	O
for	O
SRAM	B-Architecture
for	O
example	O
(	O
for	O
a	O
byte	B-Application
)	O
.	O
</s>
<s>
PRAM	O
's	O
high	O
performance	O
,	O
thousands	O
of	O
times	O
faster	O
than	O
conventional	O
hard	B-Device
drives	I-Device
,	O
makes	O
it	O
particularly	O
interesting	O
in	O
nonvolatile	B-General_Concept
memory	I-General_Concept
roles	O
that	O
are	O
currently	O
performance-limited	O
by	O
memory	O
access	O
timing	O
.	O
</s>
<s>
In	O
addition	O
,	O
with	O
flash	B-Device
,	O
each	O
burst	O
of	O
voltage	O
across	O
the	O
cell	O
causes	O
degradation	O
.	O
</s>
<s>
Most	O
flash	B-Device
devices	O
are	O
rated	O
for	O
,	O
currently	O
,	O
only	O
5,000	O
writes	O
per	O
sector	O
,	O
and	O
many	O
flash	B-Device
controllers	I-Device
perform	O
wear	B-Application
leveling	I-Application
to	O
spread	O
writes	O
across	O
many	O
physical	O
sectors	O
.	O
</s>
<s>
PRAM	O
devices	O
also	O
degrade	O
with	O
use	O
,	O
for	O
different	O
reasons	O
than	O
flash	B-Device
,	O
but	O
degrade	O
much	O
more	O
slowly	O
.	O
</s>
<s>
Flash	B-Device
parts	O
can	O
be	O
programmed	O
before	O
being	O
soldered	O
onto	O
a	O
board	O
,	O
or	O
even	O
purchased	O
pre-programmed	O
.	O
</s>
<s>
The	O
special	O
gates	O
used	O
in	O
flash	B-Device
memory	I-Device
"	O
leak	O
"	O
charge	O
(	O
electrons	O
)	O
over	O
time	O
,	O
causing	O
corruption	O
and	O
loss	O
of	O
data	O
.	O
</s>
<s>
By	O
carefully	O
modulating	O
the	O
amount	O
of	O
charge	O
stored	O
on	O
the	O
gate	O
,	O
flash	B-Device
devices	O
can	O
store	O
multiple	O
(	O
usually	O
two	O
)	O
bits	O
in	O
each	O
physical	O
cell	O
.	O
</s>
<s>
In	O
effect	O
,	O
this	O
doubles	O
the	O
memory	B-Device
density	I-Device
,	O
reducing	O
cost	O
.	O
</s>
<s>
Because	O
flash	B-Device
devices	O
trap	O
electrons	O
to	O
store	O
information	O
,	O
they	O
are	O
susceptible	O
to	O
data	O
corruption	O
from	O
radiation	O
,	O
making	O
them	O
unsuitable	O
for	O
many	O
space	O
and	O
military	O
applications	O
.	O
</s>
<s>
PRAM	O
cell	O
selectors	O
can	O
use	O
various	O
devices	O
:	O
diodes	O
,	O
BJTs	O
and	O
MOSFETs	B-Architecture
.	O
</s>
<s>
In	O
transistor-selected	O
arrays	O
,	O
only	O
the	O
selected	O
bit	O
lines	O
contribute	O
reverse	O
bias	O
leakage	O
current	O
.	O
</s>
<s>
Often	O
the	O
isolation	O
capabilities	O
are	O
inferior	O
to	O
the	O
use	O
of	O
transistors	B-Application
if	O
the	O
on/off	O
ratio	O
for	O
the	O
selector	O
is	O
not	O
sufficient	O
,	O
limiting	O
the	O
ability	O
to	O
operate	O
very	O
large	O
arrays	O
in	O
this	O
architecture	O
.	O
</s>
<s>
In	O
August	O
2004	O
,	O
Nanochip	O
licensed	O
PRAM	O
technology	O
for	O
use	O
in	O
MEMS	B-Architecture
(	O
micro-electric-mechanical-systems	O
)	O
probe	O
storage	O
devices	O
.	O
</s>
<s>
These	O
devices	O
are	O
not	O
solid	B-Device
state	I-Device
.	O
</s>
<s>
The	O
basic	O
idea	O
is	O
to	O
reduce	O
the	O
amount	O
of	O
wiring	O
needed	O
on-chip	O
;	O
instead	O
of	O
wiring	O
every	O
cell	O
,	O
the	O
cells	O
are	O
placed	O
closer	O
together	O
and	O
read	O
by	O
current	O
passing	O
through	O
the	O
MEMS	B-Architecture
probes	O
,	O
acting	O
like	O
wires	O
.	O
</s>
<s>
This	O
approach	O
bears	O
much	O
resemblance	O
to	O
IBM	O
's	O
Millipede	B-General_Concept
technology	O
.	O
</s>
<s>
In	O
September	O
2006	O
,	O
Samsung	B-Application
announced	O
a	O
prototype	O
512	O
Mb	O
(	O
64	O
MB	O
)	O
device	O
using	O
diode	O
switches	O
.	O
</s>
<s>
The	O
announcement	O
was	O
something	O
of	O
a	O
surprise	O
,	O
and	O
it	O
was	O
especially	O
notable	O
for	O
its	O
fairly	O
high	O
memory	B-Device
density	I-Device
.	O
</s>
<s>
The	O
prototype	O
featured	O
a	O
cell	O
size	O
of	O
only	O
46.7nm	O
,	O
smaller	O
than	O
commercial	O
flash	B-Device
devices	O
available	O
at	O
the	O
time	O
.	O
</s>
<s>
Although	O
flash	B-Device
devices	O
of	O
higher	O
capacity	O
were	O
available	O
(	O
64	O
Gb	O
,	O
or	O
8	O
GB	O
,	O
was	O
just	O
coming	O
to	O
market	O
)	O
,	O
other	O
technologies	O
competing	O
to	O
replace	O
flash	B-Device
in	O
general	O
offered	O
lower	O
densities	O
(	O
larger	O
cell	O
sizes	O
)	O
.	O
</s>
<s>
The	O
only	O
production	O
MRAM	B-General_Concept
and	O
FeRAM	O
devices	O
are	O
only	O
4Mb	O
,	O
for	O
example	O
.	O
</s>
<s>
The	O
high	O
density	O
of	O
Samsung	B-Application
's	O
prototype	O
PRAM	O
device	O
suggested	O
it	O
could	O
be	O
a	O
viable	O
flash	B-Device
competitor	O
,	O
and	O
not	O
limited	O
to	O
niche	O
roles	O
as	O
other	O
devices	O
have	O
been	O
.	O
</s>
<s>
PRAM	O
appeared	O
to	O
be	O
particularly	O
attractive	O
as	O
a	O
potential	O
replacement	B-Device
for	I-Device
NOR	I-Device
flash	I-Device
,	O
where	O
device	O
capacities	O
typically	O
lag	O
behind	O
those	O
of	O
NAND	O
flash	B-Device
devices	O
.	O
</s>
<s>
NOR	O
flash	B-Device
offers	O
similar	O
densities	O
to	O
Samsung	B-Application
's	O
PRAM	O
prototype	O
and	O
already	O
offers	O
bit	O
addressability	O
(	O
unlike	O
NAND	O
where	O
memory	O
is	O
accessed	O
in	O
banks	O
of	O
many	O
bytes	B-Application
at	O
a	O
time	O
)	O
.	O
</s>
<s>
Samsung	B-Application
's	O
announcement	O
was	O
followed	O
by	O
one	O
from	O
Intel	O
and	O
STMicroelectronics	O
,	O
who	O
demonstrated	O
their	O
own	O
PRAM	O
devices	O
at	O
the	O
2006	O
Intel	O
Developer	O
Forum	O
in	O
October	O
.	O
</s>
<s>
PRAM	O
is	O
also	O
a	O
promising	O
technology	O
in	O
the	O
military	O
and	O
aerospace	O
industries	O
where	O
radiation	O
effects	O
make	O
the	O
use	O
of	O
standard	O
non-volatile	B-General_Concept
memories	I-General_Concept
such	O
as	O
flash	B-Device
impractical	O
.	O
</s>
<s>
In	O
addition	O
,	O
BAE	O
claims	O
a	O
write	O
cycle	O
endurance	O
of	O
108	O
,	O
which	O
will	O
allow	O
it	O
to	O
be	O
a	O
contender	O
for	O
replacing	O
PROMs	O
and	O
EEPROMs	B-General_Concept
in	O
space	O
systems	O
.	O
</s>
<s>
In	O
February	O
2008	O
,	O
Intel	O
and	O
STMicroelectronics	O
revealed	O
the	O
first	O
multilevel	O
(	O
MLC	B-Device
)	O
PRAM	O
array	O
prototype	O
.	O
</s>
<s>
In	O
June	O
2011	O
,	O
IBM	O
announced	O
that	O
they	O
had	O
created	O
stable	O
,	O
reliable	O
,	O
multi-bit	O
phase-change	B-Device
memory	I-Device
with	O
high	O
performance	O
and	O
stability	O
.	O
</s>
<s>
In	O
June	O
2009	O
,	O
Samsung	B-Application
and	O
Numonyx	O
B.V.	O
announced	O
a	O
collaborative	O
effort	O
in	O
the	O
development	O
of	O
PRAM	O
market-tailored	O
hardware	O
products	O
.	O
</s>
<s>
In	O
April	O
2010	O
,	O
Numonyx	O
announced	O
the	O
Omneo	O
line	O
of	O
128-Mbit	O
NOR-compatible	O
phase-change	B-Device
memories	I-Device
.	O
</s>
<s>
Samsung	B-Application
announced	O
shipment	O
of	O
512Mb	O
phase-change	B-Device
RAM	I-Device
(	O
PRAM	O
)	O
in	O
a	O
multi-chip	O
package	O
(	O
MCP	O
)	O
for	O
use	O
in	O
mobile	O
handsets	O
by	O
Fall	O
2010	O
.	O
</s>
<s>
In	O
December	O
2018	O
STMicroelectronics	O
presented	O
design	O
and	O
performance	O
data	O
for	O
a	O
16MB	O
ePCM	O
array	O
for	O
a	O
28nm	O
fully	O
depleted	O
silicon	B-Algorithm
on	I-Algorithm
insulator	I-Algorithm
automotive	O
control	O
unit	O
.	O
</s>
<s>
PCM-based	O
in-memory	O
computing	O
could	O
be	O
interesting	O
for	O
applications	O
such	O
as	O
deep	B-Algorithm
learning	I-Algorithm
inference	O
which	O
do	O
not	O
require	O
very	O
high	O
computing	O
precision	O
.	O
</s>
<s>
In	O
2021	O
,	O
IBM	O
published	O
a	O
full-fledged	O
in-memory	O
computing	O
core	O
based	O
on	O
multi-level	O
PCM	O
integrated	O
in	O
14nm	O
CMOS	B-Device
technology	O
node	O
.	O
</s>
<s>
The	O
greatest	O
challenge	O
for	O
phase-change	B-Device
memory	I-Device
has	O
been	O
the	O
requirement	O
of	O
high	O
programming	O
current	O
density	O
(	O
>107	O
A/cm²	O
,	O
compared	O
to	O
105	O
...	O
106A	O
for	O
a	O
typical	O
transistor	B-Application
or	O
diode	O
)	O
.	O
</s>
<s>
Phase-change	B-Device
memory	I-Device
is	O
susceptible	O
to	O
a	O
fundamental	O
tradeoff	O
of	O
unintended	O
vs.	O
intended	O
phase-change	O
.	O
</s>
<s>
Probably	O
the	O
biggest	O
challenge	O
for	O
phase-change	B-Device
memory	I-Device
is	O
its	O
long-term	O
resistance	O
and	O
threshold	O
voltage	O
drift	O
.	O
</s>
<s>
In	O
April	O
2010	O
,	O
Numonyx	O
released	O
its	O
line	O
of	O
parallel	O
and	O
serial	O
interface	O
128Mb	O
NOR	B-Device
flash	I-Device
replacement	I-Device
PRAM	O
chips	O
.	O
</s>
<s>
Although	O
the	O
NOR	O
flash	B-Device
chips	I-Device
they	O
intended	O
to	O
replace	O
operated	O
in	O
the	O
−	O
40-85	O
°C	O
range	O
,	O
the	O
PRAM	O
chips	O
operated	O
in	O
the	O
0-70	O
°C	O
range	O
,	O
indicating	O
a	O
smaller	O
operating	O
window	O
compared	O
to	O
NOR	O
flash	B-Device
.	O
</s>
<s>
February	O
2008	O
:	O
Intel	O
and	O
STMicroelectronics	O
announce	O
four-state	O
MLC	B-Device
PRAM	O
and	O
begin	O
shipping	O
samples	O
to	O
customers	O
.	O
</s>
<s>
April	O
2010	O
:	O
Samsung	B-Application
releases	O
512Mbit	O
PRAM	O
with	O
65nm	O
process	O
,	O
in	O
Multi-Chip-Package	O
.	O
</s>
<s>
February	O
2011	O
:	O
Samsung	B-Application
presented	O
58nm	O
1.8V	O
1Gb	O
PRAM	O
.	O
</s>
<s>
July	O
2015	O
:	O
Intel	O
and	O
Micron	O
announced	O
3D	B-Device
Xpoint	I-Device
memory	I-Device
where	O
phase-change	O
alloy	O
is	O
used	O
as	O
a	O
storage	O
part	O
of	O
a	O
memory	O
cell	O
.	O
</s>
