How do SSDs [Solid State Drives] read out information? || By: Branch Education
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,
computer, tablet, and dozens of other devices.
Second, it's composed of 3 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 0 or a 1, using electrons in the charge trap.
If there are electrons on the charge trap
it’s a 0, if there are no electrons it’s a 1.
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 a value from 0 to 7, or 3 bits of information,
instead of just 0 or 1 or 1 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 3 bits of information
or 1 of 8 different values?
By the way, there are hundreds of millions
of memory cells inside just one of these flash memory microchips
and there are 18 of these microchips
inside this particular enterprise class solid state drive
from KIOXIA
It’s rather fitting that KIOXIA 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:
this insanely small memory cell is just one of hundreds of
millions of memory cells in your smartphone
which are organized 100 layers tall,
40,000 columns wide and 50,000 rows down
The overall structure is called 3D NAND.
Here’s a sheet of paper
and a 1-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.
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,
the charge trap, and the channel,
and each of these sections is separated by a 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 through the channel.
Normally the channel doesn’t allow electrons to flow through it;
in other words, the channel is normally OFF.
Electrons are there, it’s just that they can’t flow.
However, when a voltage is applied to the 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 to repeat, when reading information from a memory cell,
if the gate voltage is below the threshold,
the channel is OFF, and electrons can’t flow through the channel.
But, when the gate voltage is above the threshold,
the channel is ON and electrons can flow through it.
What we’ve covered are some transistor basics.
Which makes sense, as this charge trap flash memory cell evolved
from a floating gate transistor and transistors in general.
Now, let’s put the charge trap 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 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
and the channel is prevented from turning ON.
In other words, the charges 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,
a stronger voltage on the gate is required.
In essence, the electrons in the charge 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 trap without stored 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,
then the channel is OFF.
And the presence of more or fewer electrons
in the charge trap 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
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.
When a small voltage is applied to both,
only the channel on the left,
the memory cell with no electrons on its charge trap, turns ON.
This is because the electrons on the charge trap of the right memory cell
inhibit the gate’s ability to turn the channel ON.
Next a HIGHER voltage is applied to both 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
it holds no extra electrons in its charge trap,
and we designate this low level of
stored charge as a binary 1.
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 value stored is a 0.
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.
KIOXIA 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
triple level cell, and is the marketing term which means
3 bits of information can be stored in each cell,
and now, you might be getting an idea as to how we can store
3 bits of information and solve our puzzle.
In order to store 8 different values,
or 3 bits of information, 8 potential different levels of electrons
need to be placed onto the charge trap,
resulting in 8 different possible threshold voltages.
The level of charge on an individual 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.
And well, this memory cell’s channel has electrons flowing
through it and therefore it’s ON.
That means there are no extra electrons
on the charge trap, and a 111 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 it has a slightly higher threshold voltage, or just a
few electrons on the charge trap, and the stored value is 110.
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 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
is read simultaneously. And now, our puzzle is all pieced together.
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 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,
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 creator’s comments
in the English Canada subtitles wherein we include details on dimensions,
exact threshold voltages and other stuff.
Thanks again to KIOXIA 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 engineering concepts!
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Thanks for watching and don’t forget to consider the conceptual simplicity
yet structural complexity in the world around you.