Now, if instead you have a positive drain voltage, VDS,

then let's take 1 case where VGS is equal to 0 and

VDS is equal to 1 corresponds to this point which shows a negative current.

Why is the current negative?

Because if the drain is positive,

more positive than the gate then you have tunneling of electrons from the gate.

To the drain, which corresponds to a negative current

through this overlap region, that's why this is shown as negative.

Eventually of course, if Vgs becomes high enough,

you end up with current similar to those that you have before, and

some were in between, there is cancellation, so if we had enough

resolution you would see this going all the way down as the other current did.

Let us now take a look at the current per unit of gate area.

This is the current density nms per square centimeter.

Each of these curves is similar to one of this curve, but

the barometer here is the oxide thickness.

So we start with a rather thick oxide by modern standards, 2.1 nm.

Go 1.9, 1.7, 1.5, and 1.3.

You can see clearly here the drastic increase of current density as you make

the oxide thinner and thinner.

So, for example, if you decrease the oxide thickness from 1.7 to 1.5,

which is only 0.2 nanometers, or 2 angstroms,

you see that the current goes up by more than an order of magnitude.

And of course it quickly goes out of hand.

Once you try to decrease the thickness even more.

So we can not make the oxide extremely thin.