So before we move on to discussing the technology.
The process technology and the
manufacturing of FinFET and Trigate transistors.
I wanted to you know give you a brief
motivation of why there's a need for, Trigate or FinFET .
Or, Fully depleted SOI kind of devices. You know why.
What's the need
for moving from a planar to this architechure.
So this scematic over here it shows you conventional planner transistor, where
the source, the channel and the drain are in the same plane.
And, drawn here is, the band
diagram showing the conduction and the valence band, of a long channel,
transistor device, assuming it's nMOSFET device with n
source drain, region, and a p channel. So this would be the
end source and drain. And this would be the channel region.
So this is you know showing the band diagrams.
So for electrons present in the source region, they essentially
see a barrier for going to the drain. And when you apply a gate
field you're essentially, you know, as you keep on applying a gate field, you move.
And you know you, negate this barrier and then
electrons can now flow from your source and drain.
So this is how it manifests itself in the ID VG characteristics of the transistor
So plotted, here is essentially
the drain current in the log scale as a function of the gate voltage So, what
you see essentially as you as you apply a gate field or gate voltage and
lower this barrier for electrons to pass from the source to the drain.
What you essentially see is an exponential increase in
your drain current as your function of your gate voltage.
So two things to point out here is essentially the slope of this exponential
increase which is also known as the sub-threshold slope or denoted by SS.
And another thing to point out is the voltage
at which you start seeing this exponential increase.
So that is also known as a threshold voltage or
the Vt.
Note that this is a very simple definition of
Vt, there are more mathematical definitions of the Vt.
For, but just for the sake of simplicity, let's just define Vt: looking at this
curve the point at which we start to
see this exponential increase in our drain current.
So what happens In a conventional long-channel
transistor, when you go from a say
a drain voltage of 10 millivolt, to a drain voltage of 1 volt,
so let's say you increase your drain voltage by 100x.
Then a long channel transistor what that translates you see 100 X
increase in your drain current in your off state.
You also see a same 100 X improvement in
your drain current in your ON state. So what you
essentially observe in a long channel device is that you observe the
same increase in your current in the off and the on state.
And your sub threshold slope also
going from 10 mV to one volt, it does not change.
So say the sub threshold slope here was 90 mV per decade.
So you see the same sub threshold slope when you go
from a lower drain voltage to a higher drain voltage.
And, your threshold voltage, you know, you have
a threshold voltage, say you have a threshold voltage
of 0.1 at 10 mV. So you see the
same threshold voltage at when you have a drain voltage of one volt.
So three things are happening.
Your sub threshold slope remains the same, your Vt remains the same, and you see
a similar increase in your on current and your off current in a long channel device.
So another way to state that is from looking at this energy band diagram for a
long channel device whether you apply a drain
voltage, a small drain voltage of ten millivolt.
Or say if you apply a large drain voltage say of one volt,
you're not modulating this barrier which essentially determines your current flow
from your source to your drain.
So your drain voltage is not modulating this barrier
for electron flow from the source to the drain.
So now let's look at what happens when you have a short channel device.
Say, you have a device which has a gate length of
30 nanometers, which is typical of, you know, today's technology.
So it's a short channel device.
And plotted here is a band diagram. the conduction band and the valence band.
So this is plotted at a low-drain voltage of 10 millivolts, and this is at
a high-drain voltage of one volt. So what you
see, is at low drain voltage things look normal.
As you apply a gate field, you see a decrease in this barrier and
your flow of your electrons from your source
to your drain is still controlled by this barrier.
Which is in effect controlled by the gate field, or the gate voltage and you see a
similar exponential increase in your drain current starting at the threshold voltage.
And you see a similar sub threshold slope of let's
say 90 mV per decade.
But now look what happens if you apply a high drain voltage.
Since, your drain is now very close, it's
only thirty nanometer away from your source.
What you see is now applying a high
voltage on your drain tends
to affect this potential barrier which exists for electrons
to flow from the source to the drain.
So what you see is
a lowering of this barrier.
So what you see is essentially the drain voltage,
or the drain is inducing a lowering of this barrier.
So which is also known as, you know, drain induced barrier lowering.
Or the acroymn for this is DIBL.
So what you essentially see is that now the barrier
for the carriers to flow from the source to drain
is being affected not by just the gate field, but it's
also being starting to getting affected by the application of this
drain voltage.
So how does it manifest, this thing manifest on the IV characteristics.
So what you see is now when you apply a high
drain voltage say you apply a drain voltage of 1V
So its 100 X increase in your drain voltage.
So what you see is a similar 100 X improvement in your drain current.
But you
see a much larger increase in your off-state.
So in your off-current, you see a much larger increase.
Why this is happening is because now even though you
are not applying any gate voltage, you are reducing this.
Barrier for electrons to go from your source to the drain
because of applying this high drain voltage, so this high drain voltage
is essentially reducing this barrier.
And so you're getting a much larger increase in your off current
Another way this can be demonstrated is, you are essentially also in this graph
seeing that your threshold voltage, which was now initially say 0.1 volt.
Now it has shifted and let's say in this case it's -0.1.
So there is 200mV shift in
your threshold voltage on the application of 1V
of drain voltage. So on your drain, application
of 1V of drain voltage has reduced
your barrier, which manifested itself as that shift in your VT.
So you know one measure of this DIBL is your
shift in your VT as you apply your drain voltage.
Another thing you notice here is now,
your subthreshold swing is degraded, so initially, you need a 90
millivolt to increase your on current by one decade.
Now, your subthreshold swing has correspondingly
degraded, let's say, 120 millivolt per decade.
So there, you know, there are three things which, you know.
We can call this short channel effect. So,
what you see is in a short channel planar device, you see a shift
in your threshold voltage which is also you know you can qualify it as DIBL.
You see your subthreshold slope degrades.
So in this case it's changing from you know, 90
millivolt per decade to 120 millivolt per decade and
also you see a much larger increase in your off current.
So you see an I-off increase.
So you know, these three things are what
is commonly quantified as the short channel effects.
So what you see is increase in your, decrease in your threshold
voltage, so you know, 200 mV for application of 1V of drain voltage.
You see a degradation in your sub threshold slope
and you see a corresponding increase in off current.