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Hello, everyone.
In this lecture, we will introduce
the circuit model of MOSFET.
You've been familiar with MOSFET
since in previous Lecture 9 and the second lecture of simulation
we've known some external characteristics of MOSFET
and get some characteristic curves by simulation.
In this lecture, we will introduce how to build the circuit model of MOSFET
based on its external characteristics.
Firstly, let's intuitively learn some applications of MOSFET in real circuits.
This is a CPU.
There're hundreds of millions of transistors in CPU.
Most of them are
consisted of MOSFETs.
This is the application of MOSFET
in an amplifier of headset.
This is the application of MOSFET
in peripheral power supply circuit of CPU.
In power system
MOSFET is also widely utilized.
Although all of them are called MOSFET,
in fact applied in different situations
they are quite different in many aspects.
Firstly, Intuitively,
MOSFETs in CPU
and power MOSFET
have large difference in size.
The size of MOSFETs in CPU
is at the order of magnitude of nanometer,
while the size of power MOSFET
is much bigger,
which can be 10 centimeter or more.
Secondly, their functions are different in various applications.
MOSFETs in power system and power supply circuit for CPU
are basically used as electronic switches,
while MOSFETs in amplifier of headset
are used as analog signal amplifier.
You will understand this point
after learning the application of MOSFET in digital circuits
and Lecture 45 about MOSFET's application as small signal amplifier.
Thirdly, the internal structure of these MOSFETs
also have differences.
MOSFETs applied in low power cases
have horizontal conduction channel between its D and S.
While power MOSFETs applied in power system
have vertical conduction channel between D and S,
which can raise their voltage level.
Ok, after learning some applications of MOSFET,
we will introduce MOSFET's operating principle.
You can learn the detailed working of MOSFET
in the course of microelectronic circuits
In our course,
instead of detailed explanations,
we just use two animations to help you for better intuitive understanding
of how MOSFET works on earth
and how the conduction channel
between its D and S is formed.
Firstly, let's see
how the conduction channel is formed
when the power source is only connected between G and S,
instead of D and S.
With the increase of VGS,
you will see the forming of conduction channel between D and S,
in which current flows.
Since there's no voltage difference between D and S,
the channel has constant width.
With the decrease of VGS,
the channel is cut off.
Next, let's see the change
of the conductance channel's form
when power sources are connected between G and S, D and S both.
We can see now the channel is wedge,
which means two sides don't have same width.
Let's see how the channel changes
when we change the value of VDS
and keep the value of VGS constant.
With the increase of VDS,
channel near D side is cut off firstly.
The cutting off region becomes larger and larger with the gradually increase of VDS.
Reversely, with the decrease of VDS,
the conduction channel will be formed again.
Anyway,
the channel always has a wedge shape
with different widths on two sides in the forming process.
Similar to Op Amp,
you're not required to grasp the detailed operation principles inside MOSFET.
You're only required to grasp its external characteristics
and build its circuit model based on these characteristics,
which means to represent MOSFET's external characteristics
in corresponding operating conditions
with simple elements, such asresistor, controlled source and so on.
This is the common circuit
of MOSFET application.
We can see,
G and S terminals form a port
and D and S terminals form another port.
There're four physical quantities on the two ports,
which are UGS IGS IDS and UDS.
So there're four combinations
of them to represent its external characteristics.
According to what we learned before,
there's a layer of oxide between MOSFET's gate terminal
and source or drain terminal,
which allows no current to flow through.
In other words, IGS is always zero.
So we don't need to plot these two curves.
We'll focus on the relationships between UGS and IGS,
and UDS and IDS.
Firstly, we will learn the relationship between UGS and IGS.
You've seen
the curve before by simulation.
UGS and IDS don't represent voltage and current of the same port,
so this is called a transfer characteristic curve.
In this curve, we find
when UGS is less than some value,
IDS is zero.
The region is called cutoff region.
When UGS is larger than some value,
IDS increases gradually.
This valued is called voltage threshold of turning on, marked as UT.
When UGS is less than UT, MOSFET is turned off.
When UGS is larger than UT, MOSFET is turned on
and current can flow from D to S.
You can review
the second lecture of simulation for better understanding.
Next, we want to know
what characteristics MOSFET shows does the curve represent
when IDS is larger than zero
and MOSFET is turned on.
Then we focus on the relationship between UDS and IDS.
UDS and IDS represent the voltage and current of the same port,
the output port,
so their relationship is called output characteristic.
We get the characteristic curve between UDS and IDS
in the second lecture of simulation.
Obviously, UGS has impact on the characteristic.
The curves are corresponding to different UGS.
The bottom curve means when UGS is less than UT,
MOSFET is turned off,
so IDS is always zero.
We miy pick out one of the curves and analyse it 142 00:08:41,322 --> 00:08:44,466 which is corresponding to UGS to be 5V.
It can be represented by a broken curve for approximation.
According to previous lecture,
we've known
the straight line crossing the origin point,
the first part of the broken line,
can be represented by a resistance.
It's called controlled resistor
since the slope of the curve,
which also means the resistance, is controlled by UGS.
IDS is constant
in the above part,
so it represents the characteristic of a current source.
It's called saturation region
or constant current region.
The region represents a controlled resistor
is called resistive region.
According to previous lectures and these simulation curves,
we can see UGS has significant impact
on the output characteristic curve.
Whether the value of controlled current source
or the controlled resistance are both influenced by UGS.