storagecapabilitiesofSSDsare
romthesamesiliconwizardry
PUsandGPUs.Bymanipulatingthe
ofa waferofsilicon,etchingit and
ariouslayersofothersubstancesto
copiccircuitscanbeformed.It’sthis
ona solid-state,purelyelectrical
mediumthatmakesSSDssofast–
electronsaroundis manyordersof
defasterthanspinningdisksand
ead/writeheads.INGGATETRANSISTORS
ettoSSDsis thetransistor,butit’s
differenttypeoftransistortothe
u’llfindina CPUorDRAMchip.With
sororDRAMtransistor,it operates
pleswitch:
signalis
neterminal
rolgate),
currentto
weenthe
r terminals
ceand
s justlikeyourfingersendinga
yourlightsbyflickinga lightswitch.
g a littledeeper,what’shappening
insidethetransistoris thatwhena voltageis
appliedtothecontrolgate,it causeselectrons
tobuildupintheotherwisenon-conductive
layer(inversionlayer)thatconnectsthe
sourceanddrain.Whenthiselectronbuild-up
(charge)is highenough,it causesthe
inversionlayertobecomeconductive,
allowingcurrenttoflowbetweenthesource
anddrain.Thisis theveryessenceof
semiconductors:theycan bebothconductive
andnon-conductivedependingontheirstate.
g g ,
volatiletransistor,whichis wherethefloating
gatetransistorcomesin.
Infloatinggatetransistors,there’sa second
gatebetweenthecontrolgateandthesource
anddrainofthetransistor.Thisgateis
completelyinsulatedfromtherestofthe
transistor,sonoelectricitycanflowthroughit.
Assuch,thegateis ‘floating’.
However,byapplyinga strongenough
voltagetothecontrolgate,electronscanbe
forcedtopunchthroughtheinsulatinglayerto
thefloatinggate.Astheelectronsbuildup,a
chargeis formedinsidethefloatinggate.As
thischargeincreases,it impedesthenormal
switchingoperationofthecontrolgate(which
happensata muchlowervoltage),untilit
essentiallyblocksthecontrolgate.Crucially,becausea relativelylargevoltage
is requiredtoforceelectronsinandoutofthe
floatinggate,whenpoweris turnedoff,there
isn’tsufficientvoltagetoforcetheelectrons
backoutfromthefloatinggate,sothecharge
remains.Thisgivesusthecrucialnon-volatile
aspectofourmemory.Instead,if youwantto
changethechargeonthegate,youcaninvert
thevoltagetothegate,forcingtheelectrons
backout,whichis howthefloatinggate(or
memorycell)is erased.
Toreaddatafromthefloatinggate,
a measurementis takenofthevoltage
requiredatthecontrolgatetoovercomeINFLOATINGGATETRANSISTORS,THERE’S
ASECONDGATEBETWEENTHECONTROLGATEANDTHESOURCEANDDRAINOF
THETRANSISTOR
g effectofthefloating
onthetransistor.If a small
d,there’snochargeon
butif a strongervoltageis
eremustbea charge.This
asthethresholdvoltage.
argeis our crucialsinglebitof
wstoreinit a 1, forwhenthe
ora 0 forwhenthecellis
esa cellreturna 0 if it’s
sethechargeis negative,so
hasa highervoltagethana
chargedcell.ERASINGDATAAND
WRITEENDURANCE
Oneofthemostfundamentalproblemswith
flashmemoryis that,in ordertoaddchargeto
thefloatinggate,youhavetoforceelectrons
throughaninsulatinglayer.Thisprocess
degradestheinsulatinglayertoa pointthat
eventuallyit fails.Evenbeforethatpoint,the
chargethefloatinggatecanholdcanreduce,
makingit moredifficulttoreliablyreadthe
thresholdvoltage.
Thisdegradationoftheinsulatinglayeris the
reasonwhyflashmemoryhasa write
endurancerating.Typically,a singlecellcan
bechargedanddischargedaround100,000
timesbeforeit’snolongerusable.That’swhy
thecontrollersfor
flashmemoryareso
carefulabouthow
datais writtento
memorycells,
ensuringthatwrite
operationsareshared
aroundthedriveas
evenlyaspossible.In comparison,youcanwrite
toharddrivesectorsagainandagainwithout
long-termharm.
Anotherkeyissueis thatdatacanonlybe
erasedin largeblocksata time,becausea large
voltageis requiredtoclearthetrappedcharge.
Typically,flashmemorycellsarearrangedina
grid,wheretheentiregridis referredtoasa
block,whileindividualrowsarecalledpages.
Pagesizesareintheregionof2KBto16KB,
with 128 to 256 pagesperblock,givingyoua
totalblocksizeofbetween256KBand4MB.
Whiledatacanbereadandwrittenatthepage
level,it canonlybeerasedattheblocklevel.WHAT
Thedata
derivedf
behindC
surfaceo
addingva
it, micros
relianceo
storagem
movinge
magnitud
movingr
FLOAT
Thesecre
aslightly
onesyou
aprocess
as a simp
whenas
senttoon
(thecont
itcauses
flowbetw
twoothe
(thesour
drain).It’s
signalto
Diggin
AA SSDS?
Crucially,though,
oncepoweris turnedoff
toa conventionaltransistor,the
chargedrainsawayfromthecontrolgate
andthe inversionlayeronceagainbecomes
insulating. Inthisway,a standardtransistoris
volatile,asit can’tretainknowledge oftheon/
offstateofthetransistorwhenpoweris lost.
Forlong-termstorage,weneedanon-thecancelling
gateandswitcho
voltage is require
thefloatinggate,
requiredthenthe
voltage isknown
Thisstoredcha
data.Wecannow
cellis discharged
charged.Whydo
charged?Becaus
adischargedcell
WHAT chargedcell
There’s not a lot
going on physically
inside an SSD