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How does NAND Flash Work? Reading from TLC : Triple Level Cells || Exploring Solid State Drives | Branch Education | YouTubeToText
YouTube Transcript: How does NAND Flash Work? Reading from TLC : Triple Level Cells || Exploring Solid State Drives
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we have an engineering puzzle for you
let's set it up
first here's just one of the hundreds of
millions of nanoscopic charge trap flash
memory cells
used to store information in your smartphone
smartphone
computer tablet and dozens of other devices
devices
second it's composed of three functional
sections a gate a charge trap
and a channel with dielectric barriers
used to separate
each section third this memory cell can
be used to store
one bit of information either a zero or
a one
using electrons in the charge strap if
there are electrons on the charge strap
it's a zero if there are no electrons
it's a one and fourth solid state drives
worked this way about a decade ago so
the puzzle is
how do we increase the storage capacity
of this one
charge trap memory cell so that it can store
store
a value from zero to seven or
three bits of information instead of just
just
zero or one or one bit of information
in other words how can one memory cell
with a single charge trap
be manipulated or engineered so that it
can store three bits of information
or one of eight different values
by the way there are hundreds of
millions of memory cells
inside just one of these flash memory microchips
microchips
and there are 18 of these microchips
inside this particular
enterprise-class solid-state drive from keoksia
keoksia
it's rather fitting that keyoxia was
willing to sponsor this video
because under their company's former
name toshiba memory
they invented nand flash memory in 1987
but we'll discuss them more later
to start let's give a little more context
context
this insanely small memory cell
is just one of hundreds of millions of
memory cells in your smartphone
which are organized one hundred layers tall
tall
forty thousand columns wide and fifty
thousand rows down
the overall structure is called 3d nand
here's a sheet of paper and a one euro
cent coin
so you can get a sense of the height and
scale of these cells
let's zoom back into an individual
memory cell
this memory cell is composed of
concentric cylinders which are a result
of manufacturing these cells in vertical columns
columns
let's focus on a cutaway of the cylinder
to make it easier to visualize and
understand what's happening
as mentioned before here we have the gate
gate
the charge trap and the channel and each
of these sections is separated by a dielectric
dielectric
that acts as an electric insulator and
prevents the flow of electrons between
sections but allows electric fields to
pass through
in the center is a non-conductive
material called
core filler but it's just there for
structural support
and not much else so we're going to
remove it
the first step for solving our puzzle is
that we have to understand how this
memory cell reads out information
to begin let's simplify the design and
remove the charge trap
and we're left with a basic transistor
the gate controls whether electrons can flow
flow
through the channel normally the channel
doesn't allow electrons to flow through it
it
in other words the channel is normally off
off
electrons are there it's just that they
can't flow
however when a voltage is applied to the gate
gate
an electric field is emitted from the
gate which turns the channel on
and thus allows electrons to flow
through the channel
the minimum voltage required to turn on
the channel
is called the threshold voltage which is
a key term
that will get used a lot in this episode
understanding this concept is important so
so
to repeat when reading information from
a memory cell
if the gate voltage is below the threshold
threshold
the channel is off and electrons can't
flow through the channel
but when the gate voltage is above the threshold
threshold
the channel is on and electrons can flow
through it
what we've covered are some transistor basics
basics
which makes sense as this charge trap
flash memory cell
evolved from a floating gate transistor and
and
transistors in general now let's put the
charge strap
back into the middle to turn the
transistor into a memory cell
when we place electrons or charges onto
the charge trap located between the channel
channel
and the gate and when a voltage
just above the threshold voltage is
applied to the gate
the electrons in the charge trap disrupt
the electric
field emitted from the voltage on the gate
gate
and the channel is prevented from
turning on
in other words the charge is stored in
the charge trap
inhibit the gate's ability to turn the
channel on
in order to overpower the inhibiting
electric field
caused by the electrons on the charge trap
trap
a stronger voltage on the gate is required
required
in essence the electrons in the charge trap
trap
shift the threshold voltage of the
memory cell
by using the difference in threshold
voltages between a charge trap with
stored electrons
and a charge drop without stored electrons
electrons
we can store and read different values
if you're confused don't worry we'll
explain this concept further
but for now let's quickly recap
applying a voltage to the gate that is
greater than the threshold voltage
causes the channel to turn on thus
allowing electrons to flow through it
if the applied voltage is less than the threshold
threshold
then the channel is off and the presence
of more
or fewer electrons in the charge trap can
can
shift this threshold voltage now
let's demonstrate how we use this
phenomenon to store
information inside each memory cell
to do that let's bring back the other
parts of the charge trap flash memory cell
cell
and duplicate it so that we have two
memory cells
the one on the left has no electrons in
its charge trap
and the one on the right has a lot of electrons
electrons
when a small voltage is applied to both
only the channel on the left the memory
cell with no electrons on its charge trap
trap
turns on this is because the electrons
on the charge trap of the right memory cell
cell
inhibit the gate's ability to turn the
channel on
next a higher voltage is applied to both gates
gates
and thus the electric field on the right
memory cell's gate
becomes strong enough to overcome the
inhibiting effect of the electrons on
the charge trap
and its channel turns on as a result of
the two memory cells turning on at
different gate voltages
we can conclude that the two memory
cells have different threshold voltages
and thus have different numbers of
electrons stored in the charge trap
when a charge trap turns on at a small voltage
voltage
it holds no extra electrons in its
charge trap
and we designate this low level of
stored charge
as a binary one when the charge trap
turns on at a higher voltage
it means there are a lot of electrons in
the charge trap
and the binary values stored is a zero
details of this inverted assignment are
mentioned in the creator's comments
let's take a short break and briefly
talk about this enterprise
class solid state drive from kioxia
here's a consumer class ssd
and here's an enterprise class ssd
they look similar from the outside but
are entirely different on the inside
keoxia provides these leading quality
enterprise class
pcie nvme solid-state drives
and they can fit in the same space but
have capacities
up to a whopping 30 terabytes and use a
proprietary architecture built with
their own
controller firmware and bics flash
3d tlc memory in order to deliver
incredibly high sustained read and write
performance and reliability
by the way tlc in this context stands for
for
triple level cell and is the marketing
term which means
three bits of information can be stored
in each cell
and now you might be getting an idea as
to how we can store three bits of information
information
and solve our puzzle in order to store
eight different values
or three bits of information eight
potential different levels of electrons
need to be placed onto the charge strap
resulting in eight different possible
threshold voltages
the level of charge on an individual trap
trap
is determined by a sequence of
increasing voltages
applied to the gate and then correlating
the specific level of voltage
at which the channel turned on with the
associated memory cell
when a set of memory cells are all next
to each other
this group is called a page and the same
voltage is applied to a gate that is
shared by
every memory cell in the page let's
cover up the level of charges in the
charge trap
as if we didn't know the stored value
the voltage applied to the shared gate
starts small and we check each and every
channel to see if electrons can flow through
through
and well this memory cell's channel has
electrons flowing through it and therefore
therefore
it's on that means there are no extra
electrons on the charge strap
and a 1 1 1 value is stored there
next we step up the voltage on the
shared gate
and check if any other channels have
turned on
and we see that this channel is now on
so that means that it has a slightly
higher threshold voltage
or just a few electrons on the charge trap
trap
and the stored value is one one
zero we continue stepping up the voltage
and correlate at which voltage each
channel turns on
and that gives us the corresponding
threshold voltage
which directly relates to the level of
stored charges on each memory cell's
charge trap
and the stored binary value in each cell
this process happens incredibly fast
in order to read information from
millions of cells
and read the megabytes of data your smartphone
smartphone
goes through this voltage stepping up
cycles thousands of times every second
fortunately the memory cells are
organized in pages
and every cell in a page shares a common
control gate
and thus all information from an entire page
page
is read simultaneously and now
our puzzle is all pieced together however
however
there's another puzzle how do we add
electrons to a charge trap
in other words how do we write to a
memory cell
the solution involves materials that are around
around
75 to 100 atoms thick
as well as quantum mechanics so that
topic is covered in a separate episode
that you can find
here this episode is part of a series of
episodes that explore solid state drives
3d nand and how we save information on smartphones
smartphones
tablets or pretty much any device in
this day and age
if you want you can watch this video a
second time
and if you do we recommend you check out
the creators comments in the english canada
canada
subtitles wherein we include details on dimensions
dimensions
exact threshold voltages and other stuff
thanks again to keoxia for sponsoring
this video
additionally we'd like to thank our
youtube membership supporters and
patreon supporters
for helping in our goal of exploring complex
complex
engineering concepts if you want to help
this channel
comment below like this video and subscribe
subscribe
thanks for watching and don't forget to
consider the conceptual
simplicity yet structural complexity
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