Custom PC - UK (2019-12)

That is, only when all the
transistors are on (all the inputs
to the gate are switched to 1) will
they send a 0 signal to the bit line
(the output of the gate).
The reason NAND is still
used when talking about flash
memory devices – rather than us
just calling it flash memory – is
because there is also a different
type of flash memory that uses
a NOR gate arrangement of
transistors. Here, instead of
the floating gate transistors
being connected in series with
only one connection to the
bit line at the end, each has
its own connection directly
to the bit line. As a result, it’s
faster to read from NOR flash,
which makes it well suited
for direct code execution, as
used in embedded devices.
However, the downsides to
NOR flash are that it’s slower to
erase and write new data, it’s
more expensive to make and it
can’t reach the data density of
NAND technology. That’s why
nearly all PC flash memory
devices, such as USB memory
sticks, SD cards and SSDs, are
made using NAND flash.

3D NAND
There’s one more key
development in the physical
mechanics of modern flash
memory: 3D NAND. With this
relatively new development, instead of
transistors being formed in a single layeron
the surface of a piece of silicon, they’re
formed in multiple layers, stacked vertically.
Specifically, the whole structure of the
transistor is morphed so that the source,
inversion layer and drain extend vertically
from the base substrate. The control gate
and floating gates are then formed around
the channel that runs between the source
and drain.
This arrangement enables manufacturers
to massively increase the overall densityof
the memory cells, with the latest 3D NAND
chips packing in 96 layers of NAND cells.

What’smore,3DNANDcanprovide

improvedperformance,betterendurance

andlowerpowerconsumptionthanstandard

NAND.That’swhy,inthelasthandfulof

years,nearlyallbulkflashmemoryhas

switchedtoa variationon3DNAND.

Bearinmindthat,whencomparingthe

manufacturingtechnologiesof3Dand2D

NAND,it’snotmeaningfultocomparethe

nodesize– thetypical22nmor7nmfigures

youseeattachedtosiliconmanufacturing

processes.That’sbecausetheprocessis

fundamentallydifferent,withthefocusbeing

onpackinginmorelayers,ratherthangetting

moredetailperlayer.

CONTROLLERS

Aswithanycomputerstorage,anSSDisn’t

juststoragecells;it reliesonseveralother

componentstointerfacewithyourPC.Inthe

caseofanSSD,theseincludea portionof

high-speedvolatilememory(DDR),several

flashmemorycontrollers,a centralprocessor,

a portionofNORflashtostorefirmware,an

interfacecontroller(SATA,PCI-E)andmuch

more.Thesepartsallworktogetherto

organisehowdatais writtentoorreadfrom

thedevice,ensuringthedevicedeliversand

retainsitsdataina speedyfashionwhilealso

controllingwearlevelling,garbagecollection,

encryptionandsoon.

SLC MLC TLC

WRITECYCLES 100K 10K 5K

BITSPERCELL 123

READLATENCY

MICROSECONDS^2550100

WRITELATENCY

MICROSECONDS^250900 1,500

ERASELATENCY

MICROSECONDS 1,500 3,000 5,000

VTOTAL

TOTALVOLTAGE

CELLCAPABLE

OFSTORING

SLC

1BIT/CELL

SLC NAND MLC NAND TLC NAND

MLC

2BIT/CELL

TLC

3BIT/CELL

1

0

00

000

001

010

011

100

101

110

111

01

10

11

More charge-level

granularity increases

data density

Multi-level cells

increase data

density but reduce

performance

and lifespan