My purpose in this video is to explain to you the basics of flash memory operation.
How the device works and how you can program and erase the flash memory cell.
So the first thing that I want to do is
to compare the flash memory to a transistor device.
So if you look at the flash memory cell, drawn over here is a,
the cross section of your flash memory cell, and drawn
over here is a cross section of a normal, transistor.
So if you compare, your flash memory cell to your
transistor, it looks, very similar, you know a normal transistor has
a source, and a drain, and a gate electrode similarly,
it has, The source and the drain and a gate electrode.
This one has a gate oxide.
This one also has a gate oxide.
The only different thing or the only extra element in a flash memory
cell is you have this extra gate which is, you know, typically a polysilicon gate.
You have this extra gate which is called the floating gate.
And the reason why it's called floating is because there is no direct access to
this gate if you think about it, you know, there is always
a terminal which connects to your source and drain and your gate.
And you can directly apply a voltage to it but as compared to that this is floating
gate, there's no direct electrical contact available to this floating gate.
Also, it's it's surrounded by this this
oxide, material which is supposed to be insulating
so there is no way there could be a leakage but between
my floating gate and my and my source or my drain or
my control gate. So that's why this, this this electrode is
called the floating the floating electrode or the floating gate.
But besides that these two devices look pretty much the same, so if you
remember from your your basics are device physics, of course, and I want to borrow
an equation from there to explain the
principle of operation of this flash memory device.
So, if you remember, from your basic device
physics course, if you have a MOS capacitor.
And you have some charge which is trapped
inside the mass capacitor.
The threshold dependence is essentially of that
mass capacitor as given by this simple formula.
So your threshold voltage is your as you are essentially
the, these are the normal DOMs and this is the extra DOM which comes because of these
track charts as opposed. There was amount of charge Q which
was trapped inside at a distance Dt away
from my gate. The, the change in the threshold voltage
given by simply this Vt is is dependent on Qt
by proportional to how faraway from there and depending upon the
dielectric constant of this this oxide. And you know I can
re-arranged this answer so that e over
dt is essentially nothing but the capacitance.
This capacitance between the gate and this trap
charge and this threshold voltage is essentially given by,
this normal term, then this Qt divided by
this capacity coupling between my gate and this structure.
So if you if you carry that same analogy over here you can
essentially apply the same reasoning that safely
this floating gate is nothing but a layer to trap this charge.
And depending upon whether I have some charge trapped over here or not.
My threshold voltage
would essentially just be dependent on that trap charge by
this formula where I'll have a shift in my threshold
voltage will depending upon whether I have some charge or
whether or not and this shift would be given by.
Is this delta Vt depending upon this change in the charge
in the floating gate and the divided by this capacitance coupling.
And that is in fact the case, so I'm borrowing this from a paper and
it it shows that essentially if you have At.
No charge in your trapped in your floating gate
so this would be the floating gate over here.
And if you have no charge trapped in your floating gate, your electrons
can very easily flow from here to here and so you get,
you get essentially If you have no charge stood, you, when you apply
drain, you apply a gate voltage your device turns on early and.
So suppose you are reading at a particular voltage of VR, you get a high current.
Add that voltage.
And then what happens is if you apply a,
if you have charge stored in your floating gate.
If your floating gate is now.
Is full of electron.
It hinders this channel from turning on and it,
it prevents these, electron, from flowing from here to here.
Unless you apply a high gate voltage which,
overcomes the effect of, this, floating gate charge.
But the drift in the threshold voltage, the delta VT, is
essentially given by the amount of charge that you have in this
floating gate, divided by this capacities coupling between my floating
gate and, my top gate, which is also known as a control gate.
So, this, is essentially, so, as you can see over here, if you have a charge
trapped over here and you apply.
read voltage this you won't get any current so the threshold
voltage has shifted to higher value and you won't be able to
read any current at that particular voltage and you can say
that you know the, the, the device is in a high VT.
Hence the threshold voltage of my transistor
is affected by the amount of charge stored
in this floating gate, by this simple governing equation.
So to understand how the program, in it's behavior of this floating gate device.
It's very important to understand two things.
One is the capacitive coupling And the other one is the tunneling phenomena.
So let's start by first with the,
understanding the capacity of a coupling.
So I can relate the amount of charge into this
floating gate by these, To these different other terminals that have my
source electrode, my drain electrode, my gate electrode, by simply
writing Q equal to cv or simply writing down the maximal equation.
So what I can write down is my charge in
the floating gate, is essentially is related to these different
And my drain is at a potential of VD.
My, channel or my body, is also, also at a potential of zero.
And my gate electrode, my control gate electrode has a potential of, VG.
So what I can do is write down this Q equal to CV formula.
And this Q in my floating gate is essentially related to this, gate volt-,
gate voltages by this, capacitive coupling.
Between my control gate and my floating gate.
It's, related to my source potential by this, Cs capacitance.
It's related to my body potential by this, by this, body of our channel capacitance.
It's also related to this, Drain voltage
by placing capacitive coupling voltage between the gate.
And then I can rearrange this such that I collect
all the terms of which have the floating gate potential and
I collect all the term which have my gate potential
and I collect all the term which have my drain potential.
So if I do that so if I do that then essentially I can rewrite this equation
and this Q in my floating gate as related
to this voltage in the floating gate by these
so here are this capacitance add up and I can call the different
total of these capicitances I can label this as Ct.
So let me simply this further and I want to, what
I want to do is I want to rearrange this equation again
so that I can relate this potential on my floating gate
to the charge on the floating gate and these voltage on
the control gate and the drain voltage.
So what I do is I take all of these terms
on the other side and I can rearrange it such that my
floating gate potential is now related to my the potential I apply on the
control gate and its related by this proportionality factor.
Which is the ratio of the capacitiance coupling with
my control-gate, divided by this total capacitance,
and it's also related to my my
drain voltage potential by this capacitance with
the drain divided by the total capacitance.
And it's related to the charge on the floating gate by this charge on the
floating gate divided by this total capacitance so
this again is a very, very important formula.
Now, let me put it in a box.
And one of the key terms in this formula is this capacitor is the
ratio of this capacitance between my control
gate and my total capacitance is also known as the gate
coupling, ratio. And you want traditionally
want to keep this as high as possible. General rule
of thumb is that you at least, keep it more than 0.60.
A gate coupling ratio of 0.6 means that assuming that, you know, I have
my other terms my drain potential is 0, and my charge to start with is 0.
So those terms vanish. So my floating gate potential is
[INAUDIBLE]
related to my control gate potential by this gate coupling ratio.
And if I will get coupling ratio of point 6, that means
if I apply a ten voltage here it translates to a six
[UNKNOWN].
So if I want a more efficient programming or a faster
programming what you need is a high gate coupling ratio.
So the next thing which is, really important to understand, the operation
of, flash, memory, is this, concept, of, tunneling.
And, tunneling is essentially, you can,
there are three different, regimes, of for tunneling.
One is that when you have essentially no no potential difference
between your between your substrate and your gate.
So shown here is this band diagram which is a
[INAUDIBLE]
capacitor. So you have silicon substrate here, and
you have a gate over here. And when
you have no potential, when you don't have any electric field there's essentially
there's no tunneling current. And then you have two
different regimes. One is at a very high gate potential.
So when you apply a gate potential, which is very high you get
this regime known as fowler norden tunneling, also known as a field emission.
And, in that case your electrons which are tunneling from your
substrate to your gate, they essentially see this barrier for tunneling which is
in the face. in the shape of a triangle.
And do you this barrier?
And they can tunnel through it and its called Farther Northern Tunneling.
The gym in between, that is in between the North tunneling and this
further Northern tunneling is the, is the regime known as the direct tunneling.
So in a direct tunneling your,
your potential is somewhere between 0 volt but not as high as your falling
[INAUDIBLE]
and tunneling voltage.
And in that case your carriers which are tunneling from your, from your substrate
to your gate they see this, again, this potential barrier for tunneling.
But this barrier is now trapezoidal in our case, ten shapes.