In this video, we will discuss some short channel effect that could affect the

performance of your MOSFET and make the behavior of the MOSFET deviate from the
long channel theory that we have discussed so far. So, what is a short channel? A
typical long channel device is a device with a channel length of several microns to
about ten micron, that's considered the standard long channel, and so, anything
that's shorter than that will be considered a short channel. But more rigorously,
when you compare the channel lengths to the depletion region width between the
drain and the substrate, then you really are in the short channel regime, and there
are some effect that becomes very pronounced in those cases that affects both the
drain current in the sub-threshold and the above threshold regime. So we will
discuss those effects. First, we will discuss several effect that affects a sub-
threshold conduction here. First effect is source drain charge sharing. Now in
order to understand this, you need to look at this charge distribution figure as
shown here. So, in order to invert the channel, in order to turn the channel on and
that is to create inversion layer in this MOS device forming the channel region,
you first have to apply enough voltage on the gate to produce large enough
depletion region width. When the depletion region width reaches the maximum value,
at that point, the depletion region width stops changing and you form inversion
layer and charge carriers in the inversion layer increases exponentially as you
increase your gate voltage further. Now, suppose, oh not suppose, always, there is
a region on the side, source side and the drain side that, there is a depletion
region overlapping, why? Because your source region and the drain region are n-type
region and your substrate is p-type. So this is a PN junction. This is a PN
junction. There is always a depletion region width. So there is this depletion
region width developing due to the PN junction of source and substrate, and the
drain and substrate. If your channel length is very long, then this overlap region
is very small, and it's negligible, so you don't need to worry about it. However,
if your channel length becomes comparable to the depletion region width of this
drain to substrate junction and the source to substrate junction, then this overlap
region represents a substantial fraction of this entire depletion region width
under the channel. In that case, you now have a substantially lower volume that you
have to deplete in order to produce the inversion layer, which means that you can
create inversion layer with a substantially smaller voltage on the gate, which
means that your threshold voltage shifts become smaller. So, this is an effect.
Obviously, more pronounced in the short channel device, and the effect is shown
here, your threshold voltage. If you plot threshold voltage V sub T as a function
of channel, when the channel length becomes small, your threshold voltage goes down
substantially. This effect is more serious for a thicker oxide layer because your
oxide capacitance is lower in that case compared to the junction capacitance
between the source and drain. So, lowering the threshold voltage leads to a larger
current at a given gate voltage, so it leads to a lot more pronounced sub-threshold
conduction. In order to rigorously describe this, obviously, you will have to set
up a full Poisson's equation in the full 3D situations, and that way you can
calculate the effect on the threshold voltage more accurately, but that's beyond
the scope of this course, and we will not deal with it, but that's the way to do it
if you need to do it rigorously. The next effect that we will discuss is a drain-
induced barrier lowering, and in order to do this, we once again recall the energy
band diagram in the channel region. So once again, we invoke this energy band
diagram, 3D band diagram of this MOSFET on its side. If you once again look at the
surface region, the energy band diagram of the surface region looks like this. So
here is the source potential. Here is the drain potential when drain voltage is
zero. In the channel region, your potential is high because it is a p-type region.
We're talking about below threshold, so inversion layer has not formed yet. So
source region has a higher potential because it's a p-type region. Now, this is the
case for the long channel region. So channel region is long. When you apply a
voltage on the drain, it lowers the potential of the drain, so this side becomes
like this. But because whatever changes are happening on the drain side, it's far,
far away from the source, so the source side potential does not change. However,
when your channel is short, then the zero bias case, zero drain voltage case is
shown by this curve here. Now you apply a voltage on the drain brings this down and
that tends to lower the energy barrier on the source side. Therefore, carriers are
more easily injected into the channel region, and it leads to higher current. So
once again, it leads to a substantial increase in the sub-threshold conduction. The
last effect is subsurface punchthrough, and the subsurface punchthrough is best
depicted by these figures here showing the depletion region. So when you apply a
voltage on the drain, you are essentially applying a reverse bias voltage on the PN
junction between drain and the substrate, and when you apply a reverse bias on the
PN junction, depletion region width increase. So depletion region width increase,
increase, increase. If your channel is short, this increasing depletion region may
merge with the depletion region on the source side as shown here. When that
happens, there is a secondary current path that could bypass the channel entirely.
There is a current that could go through this punchthrough depletion region far
below the channel through the substrate. This increases the current because you
have just opened up a new current path. So, in both the above threshold conduction,
your current increases, and also, below threshold conduction, also increases
because once again, it doesn't matter whether you're above threshold, below
threshold, you have a secondary current path, and it increases the current. But, of
course, the effect on the sub-threshold conduction will be a lot more dramatic
because in the sub-threshold conduction case, your channel is off, so the current
through the channel is very small, so the effect of any additional current through
this punchthrough effect will be a lot more dramatic, makes a much greater change
in your current as shown here in this figure.