This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.
6.2 MOSFET Switches
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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 Dr.Ferri.
In this lesson we will look at MOSFETs switches.
In our previous lesson we had an introduction to MOSFETs, and
then we examined the physics of MOSFETs.
In this lesson we will examine the use of a MOSFET as a switch in a circuit.
And then we will introduce the CMOS devices, and
as I mentioned in the previous lessons, the MOSFET used as a switch is
a very common way that we use transistors in digital circuits, especially computers.
Now, what we've seen in the past is that for particular value of v sub ds, so
that's the voltage difference between V sub d and, and this, this big ground.
For particular value here, if I switch between a low value of visagy here,
a high value, I'm going to be going on these curves here.
This is a high value of high visagy and
this a value of low visagy right here, along the, the the axis.
So.
When you see these little red dots here, if V sub g is low, I've got zero current.
If v sub g is high, I'm at this current level here.
In other words, we can look at this as a switch.
In this case we say, if there's current flowing, the switch is on.
And if there's no current flowing, the switch is off.
[SOUND].
So, we can write this in a little table.
If V sub g is high, the switch is on, if V sub g is low, the switch is off.
And that basic table is what we use to analyze these sorts of circuits.
[SOUND] Let's put that to use.
And we want to analyze a simple NMOS inverter circuit.
In this case, we're connecting what we call the V-in to the circuit,
so voltage into the circuit, we're connecting that to the gate.
And the voltage out.
We've represented right here.
So, we've got a voltage in and voltage out to this circuit.
We're going to look at the two cases.
The V in is high, and V in is low.
Now, low is.
Essentially are very close to zero volts.
High, it depends on the sort of circuit we're looking at,
what the substrate is, what you consider high.
But let's just consider it as high voltage with respect to that particular substrate,
that particular material.
So, if V in is high.
I want to redraw my circuit.
V sub d is equal 5 volts.
I've got my output, if V in is high,
this being an in moss type of transistor the switch is on.
That means there's a direct connection between the output and ground.
So the output is zero volts.
So with an input that's high, my output is low.
Now if I redraw it, over here with
V in is low [NOISE] And I want to draw this switch.
Well this V in is low, the switch is off, or open.
In that case, I've got a direct connection up here to the high voltage.
Now, in this particular case, I'm going to, going to assume that,
that There's no current flowing here that I've just
connecting the voltages together here, because if there's no current drop there,
then v's of d is equal to v out, which is high.
So, in both of these cases, I went from a high input to a low output,
or a low output to high, low input to high output.
In other words, I've inverted.
The input to output from high to low, or low to high.
Well, so we've looked at just NMOS type of behaviors so
far, and the problem is that NMOS transistors are not ideal switches.
And PMOS type of transistors, which are made from doping sing, things, with a.
P wells and having an end substrate they're not dea, ideal either.
So it's commonly used, commonly done to put these together in a complimentary
manner, and this is what we call seamoss where we use the complimentary that's what
the c stands for devices using both of these types of transistors.
So on the left here.
Is our n type of transistor.
We've looked at this before, we have a p substrate.
We connect our source down to ground.
We've got our drain gate, and we've got our substrate connected to ground.
Now on the same substrate we can create a PMOS device.
And the PMOS device is created by making a deep well here.
Out of N-doped semiconductor material.
And then we have these little smaller wells in P.
So this becomes our substrate right here, and we connect our substrate to 3.3 volts.
So let's consider it here like this is 3.3 volts, and
this particular case a CMOS 3.3 volts is high [SOUND] That's
a high voltage value, that's what we consider high for this material.
So, in this case, suppose our gate was 3.3 volts.
In other words, it was also high.
So this is high.
If it's 3.3 volts, and my substrate is 3.3 volts, there's no voltage potential here.
There's no electric field.
And nothing happens.
No channel forms, nothing.
So I get no electricity.
So with a high gate voltage, I don't get any electricity.
But if I, instead, make my gate voltage low.
I have a voltage potential between the substrate and the gate, and that means
that I've got an electric field going in this direction, and that's going to create
a P channel along here of free holes, and that creates a path for electricity.
So, in the PMOS the electricity flows.
When the gate is low.
So, to summarize the behavior of these two tables.
NMOS, on the left.
When the gate voltage is high, the switch is on.
And the PMOS on the right here, it works exact opposite way.
When the gate voltage is high, the switch is off.
And similarly on the NMOS when the gate voltage is low, the switch is off.
PMOS gate voltage is low, switch is on.
So it works exactly opposite way.
So to summarize these switch behavior, on the left is the the schematics.
The NMOS, and the PMOS.
The only difference with the PMOS is that I've got that little circle there.
So that otherwise the symbols look the same.
And then these are the tables that we just looked at.
We're going to need that to analyze the circuit.
Circuit we want to analyze is a CMOS inverter circuit.
Inverter meaning the input to the circuit and the output to the circuit.
The input is high,.
The output's low.
The input is low, the output is high.
This is similar to what we looked at before when we had a resistor in there.
Why did we build it this way instead?
Well, because this way it's going to work better.
Because the CMOS uses the p-type of transistor, and
the n-type of transistor in a complementary way.
Each one works well in a certain.
Regime. And we put 'em together,
I get a pretty good inverter.
Much better than I did if I practically, for
building that other inverter, with the, with the, resister in it.
This one works better.
So, to draw the, the circuit, if V-in is.
Low or V in is high, with CMOS that's close to zero volts and
that's close to 3.3 volts if it's high.
So, let me draw my circuit.
If V in is low,.
That means my PMOS transistor is on, and my NMOS transistor is off.
That means if I've got a V's of ss is high,
my V out is also high [NOISE] So I've a low input I get to high output.
Similarly if V in is high, well in this case my p type of transistor is off.
But my end type of transistor is on.
[NOISE] In this case my output is directly connected to ground.
Or it's low.
So that's where we get the inversion from high input to low output or
low input to high output.
And again we use a CMOS complimentor because the P type and
N type transistors each on their own are not perfect, or not very ideal switches,
but working together they do behave in a much more ideal way.
So in summary, we've examined the use of the MOSFET as
an electrically controlled switch in a circuit.
And then we introduced the CMOS for
complementary p-type, and n-type of transistor behavior.
And that is actually the type of transistor circuits that
we use in computers,
we use them in a CMOS configuration where we complement the p-type and the n-type.
And then, we also introduced inverter circuits.
In the next lesson, we will examine CMOS Logic Gates.
Thank you.
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