If we know the gate charge per unit area and the inversion layer charge per unit

area. And we typically do depending on the

model we are using. Similarly, we find a corresponding

equation for the bulk charge and for the inversion layer charge.

And because, we have split inversion layer charge into a drain charge and the

source charge, we also have the corresponding equations derived in the

same way for those charges. Now, for saturation, we do the same thing

that we did with the current. QI will be given by an expression in

terms of the terminal to terminal voltages, in this case it's VGB, VSB, and

VDB, as long as VDB is less than the pinch-off voltage, so we are in

non-saturation. And once we enter saturation, we simply

replace VDB by its maximum value non saturation being the pinch-off voltage

value V sub p. Let us now take an example, we take the

simplified source reference from inversion model.

I remind you that we have found a very simple way to write the current.

Now, by defining that parameter theta, in this theta was called the degree of non

saturation. It was one, when VDS is zero, and it went

to zero when VDS reaches saturation value VDS prime.

And then, it stays at 0 for VDS, larger than VDS prime, over here.

And we showed back then that we can write the drain source current in both

saturation and non saturation using this simple equation.

So, if you use this either to simplify the algebra, which by the way is quite

long, and it is described in the book, you find for example, that the inversion

layer charge, total inversion layer charge, is given by this expression

depending on the value of VDS. You find the corresponding value of ETA,

and from that you find the corresponding QI using this, and similarly for the

other charges. So, for this device, if you evaluate the

charges in this way and again, the details are in the book you find that the

charges versus VDS look like this. So let's take one of them for example,

let's take QB. QB becomes more and more negative, why is

that? Because as you increase the drain

potential, you increase the reverse bias, the effective reverse bias between the

channel and the body so that the depletion region widens and overall it

contains more negative charges than before.

That's why it becomes more and more negative.

Now, why does QI become less and less negative as you increase the drain

voltage? The reason is, as you increase the drain

voltage the reverse bias in the channel here makes the inversion less heavy here.

You go towards pinch-off, so the overall inversion layer charge seems to have less

and less negative charges here it becomes less and less negative, like this.

And then, you split QI into QD and QS and corresponding components are shown here.