SPEAKER 1: So let's take a closer look at what the MOSFET
device really looks like when we apply various kinds of
voltages to it.
So let's take the MOSFET, and I'm going to try to view it by
thinking about it as a pair of ports.
So I'm going to take my source port, my source terminal.
I'm going to draw it out like that.
I'm going to take the drains terminal
and draw it out there.
Take the gate terminal and draw it out here.
So notice that viewed from here, I see a port.
A port is a terminal pair.
So a port is a terminal pair.
And so here, if I have a D here and an S, here-- so I'm
going to call this voltage VDS.
I can apply a voltage VDS here.
And if I apply voltage VDS, let's say I get some current
iDS flowing through here.
I'll tell you in a second why I call it iDS.
So this is my gate terminal.
And between the gate terminal and the common source, let's
say my voltage VGS.
OK, so I have a voltage VDS from drain to source and a
voltage VGS from gate to source.
And I built you this little pattern, where I take the
common source terminal and I build a pair of ports.
So one port between drain and source, and a second port
between gate and source.
So these terminal pairs are called ports.
And here, let me say I have some current, iG.
Now, for all of 6002x, current iG will be taken as zero.
We are never going to have any current flowing into the gate
terminal in most of 6002x.
And in the cases where we do, we will
explicitly tell you so.
So in other words, what this is saying is that our common
model of the MOSFET is one where there's an open circuit
between the gate and either the drain
or the source terminals.
And so therefore, when I apply a voltage VGS, I'm going to
have a zero current flowing through it.
So this in part explains why I call this iDS and not just iD.
Because if the current from the gate is zero, then all the
current that comes into the drain must be coming out of
the source.
So all of this circuit must be flowing through here, because
the current through the gate is zero.
OK, so that is why I'm comfortable calling that
current iDS.
So now let's try to understand the behavior of the MOSFET
transistor.
So the way it works, much like we did for swithces, is look
at two cases.
So the one case is where VGS is less than some threshold
voltage, VT.
OK, so here's my voltage, VGS.
And I'm going to apply a voltage that is
less than some VT.
So VT, for example, might be 1 volt.
Between a system where a VS, the highest is 5 volts and the
low is 0 volts, my threshold voltage, VT, might be on the
order of 1 volt per instance.
So here, if VGS is less than VT-- so just as an example,
let's say it's 1 volt.
If VG is less than 1 volt, then I'm
going to get one behavior.
And if the VGS is greater than or equal to VT, I'm going to
get a different behavior.
So the MOSFET behaves like so.
So if VGS is less than VT, the MOSFET behaves like an open
circuit switch.
OK, so you should have guessed this by now.
So the MOSFET is in what we call an off-state.
So G is here.
And this is VGS less than VT.
And the MOSFET is in its off-state.
OK, my VGS is the voltage between the
gate and the source.
Now when VGS is greater than VT, what happens?
So in this situation, this is my drain.
In this situation, I get a short circuit between the
drain and the source.
And I can have a nonzero current iDS
flowing through it.
And this is my gate.
And this is when a VGS is greater than or equal to VT.
So what do we have here?
We have something that largely is very
reminiscent of our switch.
Where VGS is less than VT, I get open circuit behavior.
And when VGS is greater than or equal to VT, I get a short
circuit behavior between the D and S terminals.
This is exactly like our switch.
And in fact, the MOSFET very commonly behaves very nearly
like an ideal switch in this abstract rendition of ours.
OK.
So in this case, if you have taken a very simple view of
the MOSFET and view it like a switch, we call this the
switch model, or the S model of the MOSFET.
Here, we take this device to MOSFET and
view it as a switch.
Now, you can go and check out the internal structure of the
MOSFET in Section 6.7.
And as we go through various additional lecture sequences,
we will notice that the MOSFET is quite a bit more complex
than a switch.
The MOSFET is actually more complex.
However, as engineers, we need to learn to deal with
abstractions, and we're taking a very
simple view of a MOSFET.
If you're saying, look, it's going to behave like a switch
in its more simple form of behaviors.
And so, let's continue with the MOSFET behaving as a
simple switch and see what we can get using that behavior.
OK.
So as the next step, what I'd like to do is I'd like to do a
little demo for you and take a look at the MOSFET device on a
scope and convince you that it really behaves like a switch.
And as before, I'm going to treat it like a two-port.
One port is the DS port.
This is the D and the S, and this is my gate.
One port is the DS port, and one port is the GS port.
And S port is taken as common between the
drain and the source.
So what I'd like to do is in this demo of ours, I want to
plot and show you that [? ID ?]
characteristics of a switch.
And I'm going to plot for you the following.
I'm going to plot iDS versus VDS.
OK, so between iDS and VDS, this looks like a
two terminal element.
However, it's a funky two terminal element.
OK.
So I'm going to plot VDS versus iDS for that port.
But what I'm going to do is I'm going to plot that a a
function of VGS.
OK?
So as I change VGS, my iDS versus VGS, the plot is going
to look different.
And in fact, I'm going to get two values.
One is for VGS is greater than or equal to VT.
And a second value, second curve for VGS less than VT.
So look at what I'm doing here.
You haven't seen this before.
So I'm showing you the iDS relationship between the drain
and source terminals of the MOSFET.
You've got three terminals.
So I can plot VDS versus iDS.
And I'm going to show you what VDS versus iDS looks like for
various values of VGS, which is the gate to source voltage.
And notice I could very well have drawn a third axis with
VGS and drawn a plot that way.
But more commonly, we look at the terminal pair, the DS
terminal pair with the voltage VDS across it, and look at the
current behavior for different values of VGS.
In particular, I'm going to look at a VGS value which is
greater than or equal to VT and a VGS value which
is less than VT.
So as a switch, I have two states.
I'm on or off.
And so therefore, we get two curves for these two states.
So then, VGS is less than VT.
And remember, the MOSFET behaves like an open circuit.
OK, so VGS less than VT, the MOSFET behaves
like an open circuit.
So in an open circuit, my current is going to be zero no
matter what VDS is.
So in VGS less than VT, my current is going to be zero
and my curve is going to look like this.
OK, this the curve of an open circuit.
Let me give you a few seconds to think about what the curve
would look like for VGS greater than VT.