Another thing, which I can plot then is my my charge buildup as a function of time.
And the charge is nothing but integral of
the differences between my incoming and outgoing currents.
So starting from T equal to 0, I see an increase in my charge and then I.
When my incoming current starts to go down and my
outgoing current starts to increase. And they eventually balance out
each-other. And I see what I see is at t equal to the
saturation time. I see a saturation in my charge
build up. So this is how I hope you know, you have
understood how a flash memory operates. What you see is essentially as,
as you keep up building charge this potential on my floating gate.
But remember these two points are fixed, the
control volt, the control gate and my silicon.
So this essentially keeps on rising and that leads to decrease in my
field across my tunnel dielectrics. So my incoming current
decreases at the same time, my gate
voltage across this blocking dielectric increases.
So my outgoing current increases and eventually they
balance out each other and I get saturation.
So remember I said I need a thick I need a thicker
thicker inter-poly dielectric as compared to the tunnel dielectric.
So what if I had a thinner dielectric?
So, if I had a thinner dia, thinner IPD, so let's say this is for thickness of,
so this curve is for IPD thickness of let's say, 17 nanometer.
If I, instead of that if I had
interpoly dielectric thickness of let's say only ten nanometer.
What will happens is that I will get a much higher Jout so this saturation will
occur at a much
smaller level And I will get a much lower build up of my
charge and since my charge is directly related to my to my change in threshold voltage.
By this simple formula I will get a much lower shift in threshold voltage.
So to warrant a large change in threshold voltage with programming.
We want to keep our tunnel oxide thickness
thin but we want to keep our interpoly dielectric thicker.
So the other
[COUGH]
thing that comes next big question to ask is, you know Okay so
I have stored this charge in my in my floating gate but how do I retain it?
So a key metric for all non volatile memory is that retention.
That is how long you can retain the state of your cell
So, let's look at, you know, again let's come back to our friends
the tunnel equation and my capacitive coupling.
So, what happens when I'm in the retention state?
So when I'm retaining the charge I don't apply any voltage
on my control gate, so my control gate is essentially at 0.
All I have is build up of charge in my floating gate.
So there's already some electrons in my floating gate.
So because of that I get a negative voltage on my floating gate.
So let's say
when I have no No no voltage applied and I have electrons stored.
Maybe this floating gate will rise up to a voltage of let's say three electron volt.
So, what I need from my friend, this tunnel equation is that.
At this three electron volt, my tunnel current both across my IPD and
also both across my my tunnel oxide. They are they are
you know, small enough such that all of these electrons they don't leak out.
And there have been traditionally the requirement for retention on flash memory
cell has been that, if once you program it you need to retain it for ten years.
Now this requirement has relaxed out a little bit and now what people if you say
that you can retain it for one year in They are quite okay with it because
a well done, system limit. But still this retention is a geometric.
You don't want to take a photo on your IPhone and then
find out that, you know, a year later, it's a no longer there.
So there's this.
It's import to, you know meet this retention requirement and this, this
behavior of my tunneling where I get low tunneling current at low voltages
is key.
in maintaining and meeting this retention requirement.
The final thing I worry about is how do I, how the heck do.
Do I erase the cell so you know I've got
these electrons in but how do I get them out?
So what voltage do I apply to my
to my control get to get rid of these electrons?
So the answer is very simple.
I get What I
do is I essentially apply a large negative voltage on my on my control gate.
So I can apply a voltage of let's say -15 volt on my control gate.
And since there's already a build up of electrons in my
floating gate, it it pushes this voltage even more negative. So I get a very large
negative voltage on my floating gate.
So it moves up in a potential and move up in this a band diagram.
And what I get is essentially it now this electrons can
tunnel out of this tunnel dielectric. Or the other way to think about it is that
your holes can tunnel from your silicon into your floating gate.
and both of what it leads to essentially, the charge in the floating gate,
again, reduces back and goes down and goes back to, 0.
And you can dry, draw, that similar band diagrams, as we did in the program case.
And I'll I'll you can be assured that in
your next problem, in your next problem set, we will,
ask you to draw some of these band diagrams and these band diagram are key
to understanding and building up your knowledge in the field of flash memory.
In general, if you are, you know, if you
are in the semiconductor field always help to draw this
band diagrams and then I'm reminded of these quote from
Herr Kromer He said that, you know, if you're in,
if you're dealing with a problem with semiconductor.
And, if you can't draw the Energy Band Diagram
then, you don't know what you are talking about.
That corollary of that is if you can draw a diagram and
if you don't then your audience won't know what you are talking about.