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  • 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.

My purpose in this video is to explain to you the basics of flash memory operation.

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B1 gate floating voltage tunneling coupling charge

Baics of Flash Memory Operation: Part 1

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    陳震寰 posted on 2015/05/01
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