This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.
6.1 Introduction and MOSFET Physics
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From the course by Georgia Institute of Technology
Introduction to Electronics
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From the lesson
MOSFETs
Learning Objectives: 1. Develop an understanding of the MOSFET and its applications. 2. Develop an ability to analyze MOSFET circuits.
Meet the Instructors
Dr. Bonnie H. FerriProfessor
Electrical and Computer Engineering
Dr. Robert Allen Robinson, Jr.Academic Professional
School of Electrical and Computer Engineering
Welcome back to electronics this is Doctor Ferri.
We're starting module three on transistors.
This lesson we will do an introduction to transistors and
look at a particular transistor that is a MOSFET.
Our lesson objective is to introduce the uses of transistors.
And then also to investigate the physics of MOSFETs.
The type of transistors that we will be examining in this module include MOSFETs
which stand for metal-oxide-semiconductor silicon field-effect transistors and
also BJTs which stand for bipolar junction transistors.
Now MOSFETs are very,
very commonly used now, in fact probably more commonly used than BJTs.
So the common uses of transistors.
As switches, in particular electrically controlled switches.
This is how we used them in computers.
You hear of transistors being used in computers.
They are used as switches to build up digital circuits.
Within the co-
Within the computer.
Transistors are also used as amplifiers.
We've talked about op amps in the past, and
we said that op amps are made of transistors.
Well, we use the transistors in there to help us build an amplifier.
We also use resistors,
transistors to give us a resistor with a value that is electrically controlled.
You think of a device like a potentiometer.
Potentiometer is something that you can physically turn the value or
use a slider or use a screwdriver or dial to turn the value of
the resistor physically to do that mechanically to do that.
Well a transistor can be used as a resistor that
varies by having an electrically controlled signal to vary that resistance.
Now lets see how it behaves.
In this particular case I'm going to have, I'm going to be varying V sub G and
seeing the effect the behavior as I vary V sub G.
And in this particular case I'm, I'm going to say that V sub DS which is a potential.
From the ground, from the source to the drain.
So it's this voltage that's applied here.
We're going to say, we're going to look at the case where it's small but positive.
And VG is positive.
Now remember, a P-type substrate has extra holes in it.
It's been joked with, with holes.
When I create a, when I gain this a, a potential difference across here,
from the gate, and this is positive, I'm going to create an electric field here.
And that electric field from positive to negative is going to
force these holes down toward the, the ground.
And at the same time, it's going to pull in these, this material over here,
the N-type of channel, and I'm going to be pulling in electrons from here and
here to create a channel of electrons across here.
And that channel allows electricity to flow.
So, now we have free electrons.
And a complete channel for
it to flow, a path for it to show, to flow from the source to the drain.
That means our current, which is a flow of positive charges,
will flow in this direction.
Now, if I replace that, if I increase the voltage here.
A larger V sub G, I get a larger electric field.
And as a result I get a bigger channel.
So, the current will flow more easily.
And we can represent that by looking at the IV characteristic here.
Now this current, is a current flow [NOISE] from
the drain to the source so it's a current flow here.
The bigger the channel, the easier it is for the current to flow.
So as I vary V sub G, then it becomes easier, as I increase it,
it becomes easier for the current to flow.
It's kind of interesting to contrast this with the IV characteristics of a resistor.
So if this is, were a resistor, it would have this sort of characteristics,
linear characteristics, a line right there.
Now, this is non-linear.
It is non-linear curve.
And it, you can almost think of it like a,
a non-linear resistor, as I vary V sub G, and I look at the current.
Now, this is a particular case where I'm varying V sub G, and V sub Ds is small,
and I get this sort of behavior.
So let's look at the situation where we keep V sub G constant.
It's positive but at a constant value and instead we're going to vary V
sub Ds from a small amount to a large amount just looking at positive values.
When V sub D is small and I'm going to
represent everything actually on this curve right here, V sub D is small.
We have small current flow, we have a,
a, electric field that is going in this direction it creates a constant channel.
And this current remember is a current [NOISE] the current flow from the drain
to the source, in other words it's the current flowing in this direction.
With a small value of D sub S, we have a channel there.
That allows the current to flow.
Now what happens when I increase V sub Ds?
Is that our electric field is no longer constant it's no
longer uniform like this because I now have a voltage there.
So I get electric field that looks kind of like this.
It's not uniform and what happens is that with a small value of
V sub Ds I had a constant channel so this I'm operating in this region right here.
Constant channel.
Or close to constant channel [NOISE] and
when I increase it, V sub Ds more, I start to get pinch off.
And that's when this happens right here.
This is when the channel [NOISE] pinches off.
If I increase V sub Ds more, what happens is I, it has completely pinched off and
it looks like current’s not going to flow anymore because I don’t have this path for
the electricity to flow.
But we have to go back to our last lesson on pn junctions to remember that
anywhere we’ve got a an N and
a P type semiconductor next to each other, we create a depletion region.
And so that depletion region actually looks like this.
And electricity can jump across, or
current can jump across this depletion region.
Electrons can jump across it, but they're restricted.
So that's when we're over in this region over here.
And I'm going to call this a saturation
region [NOISE] and that's where the,
electrons [NOISE] jump across a depletion region.
[NOISE] So we've got these different characteristics,
different regions, the, in this region right here,
it's really kind of a linear region.
It's almost a constant channel.
It starts to change.
Right here is when it pinches off.
And over here is when we're operating through, with a depletion region.
So, let's summarize the behavior of the MOSFET.
And I show it in terms of this big grid,
because we actually have two variables that we're changing.
We are changing V sub G.
We're making it,
you know, from positive, but we're making a small value to a large value.
And we're also changing V sub Ds from a small value to a large value.
And, we can actually change these in a continuum, but
I'm just showing this in a grid.
Showing, like, for three discrete values.
Small, medium, large, in each case.
And I want to show you what happens physically to the, the structure and
what happens to the, the curve, the IV curve in each case.
Let's look at a case for V sub G equal to small.
V sub G equals small, we have a small channel here.
And we're on this part of the curve.
We have small channel meaning small current flow.
Small, small current flow over here.
V sub Ds is small we're right here operating where the red point is.
And over here we're on, kind of this part right here.
And as I make V sub DS large I move over.
To this point of the curve.
Now I show this as different parametric curves.
This one right here is a curve for V sub G medium.
And this top one is a curve for V sub G being large.
So let's follow along for the V sub G medium curve.
We've got a thicker channel than we did before.
Again, V sub G.
Increasing means we got a, we have thicker channels, so
you can see it in all of these channels.
V sub G increasing we've got thicker channel means more current flow.
So V sub G medium, I follow this curve, as I vary V sub Ds.
V sub Ds is small on my linear region, as I increase V sub Ds,
I'm on the sort of transition region here.
When I search pinch off and then when I've completely pinched off,
now I'm in the depletion region, I'm off on this part of the curve.
Always following this parametric curve for a particular value of V of G.
If I increase V of G to make it really large,
then I'm going to follow this curve right here, the top curve.
Again, V sub D small, I'm in the linear region.
V sub D, and large enough so that I'm near the pinch off,
then I'm in this region right here, kind of the transition region.
And with V sub D equal very large, where I've got a depletion region,
I'm in the saturation region.
So what's interesting about this is I can build devices that utilize either one
of these two characteristics it can utilize the behaviors
I vary V sub G and the current changes and I can go from curve to curve.
Or, in our circuit we can build a, build circuit that varies V sub D,
V sub Ds, and I'm going to be moving along this part of the curve, right there.
So, in the remainder of this module, we'll first look at MOSFET switches.
And then we'll go on to look at circuits in which we use MOSFETs as amplifiers.
And then finally we'll cover the other type of transistor that we mentioned,
which is a, a BJT.
And I want to, to encourage you to go to the forums and ask and answer questions.
Thank, you.
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