So far we have ignored the leakage of the drain, to body unction.
With one exception, the presence of hot electron effects that gave rise to a
drain body current. But there are other effects that can
cause even more severe problems, which can be present even when hot electron
effects are avoided And this video is devoted to such leakage currents.
I begin with a fact called Band-to band tunneling.
In conventional modern processes you have very heavily[UNKNOWN] Substraight, and
even moreso, drain. As you may recall, very heavily doped
regions are associated with narrow depletion regions.
So the bands, bend, very much, over very short distances They, schematically I
show you what happens here. And there is a region here, a narrow
region, that it kind of behaves as we saw for thin oxides.
Just like there was direct tunneling there Here there is the so-called
band-to-band tunneling where an electron can, can pierce the barrier and find
itself on the other side. So you can have current between the p and
n regions which, of course, manifest itself.
As the[INAUDIBLE] scarring between drain and body this is called band to band
tunneling. The current is a function of the peak
field in the junction which is expected to be high because of the large band
banding. And you can find expressions for this
field in the book. And it can be large, for example, 1
ampere per square centimeter. You also get a leakage current that can
be effected by the gate field, and this is called the gate induced drain leakage.
Let's see what this is. I will start with a transistor which is
on and will, operates normally. We will use this to compare it with a
case I'm about to show you. So, we assume we have plenty of charges
on the gate and below it we have a depletion region and an inversion layer
which creates the cause the drainage source current to flow and we have a
depletion region throughout like this. Now let's assume that instead of the
device being on, it is off and it is in accumulation.
So, we have negative charges on the gate, which attract positive charges in the
bulk. In other words, the p dark region, has
extra holes near the surface, so it is in accumulation.
Now, the extra positive charges there Act as if you had a more heavily doped p
region than you have over here. Now the heavier the doping of the region,
the more narrow the depletion region becomes.
So here, you see that the depletion region is more narrow than it is over
here. Now in as an exception to what we usually
do here. you see the depletional region drawn both
on the peace side and on the inside. Over here, because the peace side is much
more likely dark then the end region. Most of the depletion region extends from
the peace side.The, a very narrow part of the depletion region extends to the
inside. But over here, because you effectively
have a highly-doped p region, a significant part of the depletion region
extends into the n drain. So the drain region gets depleted over
here. Here.
Because of the narrow depletion region here, you can have high fields.
And in fact, if you are deeper into accumulation, let's say we have even more
negative charges on the gate, then these negative charges can repel the electrons
that used to exist Near the surface in the drain region.
And you get the depletion region in the N plus, the heavily doped anti region over
here. Now you have, electric fields that can be
high, and are around this region. Because of this, you can generate whole
electron pairs. For reasons we have already discussed,
the electrons find their way towards the drain being attracted by the positive
potential there. the holes find their way into the bulk
and they give rise to a current in the bulk So now you have a current between
drain and bulk because of this and as you can see it was the gate that caused the
situation there. And as you can expect, by varying the
gate voltage, the, you, you will see that the leakage varies as well.
So the large fields, cause band to band tunneling currents.
And one other thing that happens if you have traps in this region at the
interface, we talked about traps before those traps can affect the tunneling
current. So this is called trap assistant, trap
assisted tunneling. The current as we saw before in other
situations, depends on the peak field in the GIDL region.
And it can be estimated by using a two through the two dimensional analysis.
An example is given in the book. Here is an example of a drain current
versus VGS. It has a normal behavior from source to
moderate to weak inversion. Because this is a log axis and weak
inversion, we have a straight line. And then, without GIDL we have liquids
that goes like the broken lines and if you include the GIDL effect it goes like
the solid lines. So it can be very severe.
It can increase the liquids current here by several or magnitude and one warning
sometimes designers try to reduce liquids current by making the threshold larger.
And they can make the threshold dynamically larger.
By increasing VSB or effectively for a ground source by making the body more
negative. But when you make the body more negative
you worsen GIDL and you may end up with the opposite of what you thought.
Instead of decreasing leakage you increase leakage.