Hello everyone, I'm Typhoon and welcome to another lecture.
In this lecture you will learn about the mOSFET.
The most widely used transistor in modern computer logic circuitry is the mOSFET here.
Which stands for metal oxide semiconductor field effect transistor.
Among the different types of available, the most commonly used for switching purposes is the enhancement
mode mOSFET.
Now in this lecture, we will focus on understanding the behavior of this type to build a strong foundation,
and the more advanced variations will be reserved for future lectures.
So MOSFETs are typically made from silicon, a material known as semiconductor.
A semiconductor allows electricity to pass through, but not as easily as metals, and to enhance its
conductivity.
Silicon is doped, and this means intentionally introducing impurities, and depending on the type of
impurity, the silicon will favor the movements of either electrons.
Or holes.
So electrons are basically negatively charged, and the holes are effectively the absence of electrons
behaving like a positively charged particles.
Now when a semiconductor conducts mainly via electrons, it is called the n type, and when it conducts
primarily through holes, it is called the p type.
Now, the region through which the current flows in a mOSFET is called the channel.
And this channel is situated between two terminals, the source terminal and the drain terminal.
And the third terminal, called gate, controls whether or not current can flow through this channel.
Now this gate is made from the opposite type of semiconductor than the channel and the separated by
a thin insulating layer.
Now, as I explained, there are two basic types of MOSFETs the n channel and p channel.
In n channel, the source is connected to zero volt ground And the drain is connected to five volts
via a resistor.
And the p-channel.
The source is at five volts, and the drain is connected to zero volts via a resistor.
So it's basically the opposite side and you will see how big a difference this will make here.
Now these diagrams represent simplified versions of actual circuits and meant to introduce the functional
principles of mOSFET, uh, behavior.
And um here you can see the each mOSFET has three terminals.
The gate which is the input terminal controls the switch.
From here the drain Connects to the next stage in a circuit.
This is the output terminal and the source connected to the either the higher or lower potential, depending
on the mOSFET type here.
If the source is zero volt connected to zero volt and the in the p channel, we have a source which
is connected to what?
To five volts.
Now the voltage difference between the gate and the source determines whether the mOSFET is on or off.
In n channel and p channel, MOSFETs work in a complementary way, similar to how we understand complements
in a Boolean logic.
So when used together, they can create faster and more efficient digital switches.
Now we will start explaining.
Firstly the N-channel mOSFET operation.
Now we will.
Let's first focus on the N-channel mOSFET shown here.
The drain is connected to what?
Connected to plus five volts through a resistor R.
And the source is connected to zero volts, which is ground.
When the voltage is high, like five volts relative to the source, the channel becomes conductive and
the mOSFET basically acts like a closed switch.
As shown here.
And the voltage at the drain here drops to zero near zero.
However, the current must pass through the resistor R here, which consumes power by converting electrical
signals Animals into heat, and this is not ideal because it leads to unnecessary power loss.
And when the gate voltage is low, which is zero volts, let's say relative to the source.
Now the channel is not conductive.
So the mOSFET behaves like an open switch like this.
So no current basically flows here.
And the resistor acts as a pull up resistor.
Raising the voltage at the drain to five volts.
Now this behavior is essential for digital logic since it helps represent binary states on.
In this case one is zero volt.
But the off here is five volts, which is weird.
And you will.
Understand why it is like that.
And here we have the P channel.
Now let's examine the p channel mOSFET.
Now here we have the opposite.
The source is connected to five volts.
And drain is connected to zero volts through this resistor R.
And when the gate voltage is equal to the source like for example, if the gate voltage is five volt
and the source is five volts here.
The mOSFET is off.
So it behaves like an open switch.
No current flows through the resistor like that.
And when the voltage is lower than the source for example let's say zero volts.
The channel becomes conductive and the mOSFET acts like a closed switch.
So current flows from source to where where.
drain right and the voltage at the drain rises, and in this case the resistor functions as a pull down
device, ensuring drain is at a low voltage when the mOSFET is off.
So summary of the mOSFET switching.
Is mOSFET act like a controllable switches basically.
So n channel we have here in the mOSFET, turn on when the gate is higher voltage than the source.
And the p channel here turns on when the gate is at a lower voltage than the source.
Using both types together enables faster switching and better power efficiency, especially in complementary
MOS, which are CMOs logic design.
Although the pull up and pull down resistors help set proper voltages at the drain, they introduce
two major problems.
The first is power consumption.
So when a mOSFET is on, current flows through the resistor, in this case here, uh, which as you
know, resistors are not very efficient.
So the current is wasted as heat.
And even though mOSFET gate draws almost no current in a steady state, a short burst of current is
needed to change its state.
And this current typically comes from the drain of a previous, uh, mOSFET, and which is limited by
the resistor in its path.
And if the resistor value is high, which to basically reduce the power, the less current flows and
the switching becomes slower.
Now this creates a trade offs.
Uh, if you use small resistors it will allow faster switching, but it will consume more power.
And if you use the large resistors, it will reduce power consumption, but it will slow down the system.
So basically in a high speed computer logic, finding the right balance between speed and energy efficiency
is critical, as you have learned so far.
And this dilemma ultimately led to the development of resistor less logic families like c MOS, where
pairs of MOSFETs are used together to eliminate the need for pull up, pull down resistors entirely.
Yeah, that was that was it with our lecture.
And in the next lecture we will explore how these transistors are combined to form the building blocks
of logic gates like and or and not.
And how the switching behavior we have studied so far translates to Boolean algebra operations in the
hardware.
Now thank you for watching.
I'm Tiffany and I'm meeting you in the next lecture.