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.
