- [Instructor] Unlike Bipolar Junction Transistors
which work by biasing a pair of P-N junctions,
Field-Effect Transistors turn on and off
by using an electric field to control the behavior
of a semiconductor material,
making it more or less conductive.
There are two main types of Field-Effect Transistors,
Junction Field-Effect Transistors referred to as JFETs
and Metal-Oxide Semiconductor Field-Effect Transistors
or MOSFETs.
There are a lot of similarities between JFETs and MOSFETs,
but MOSFETs are more widely used,
because they have a higher input impedance than JFETs
so for this course, I'll be focusing primarily on MOSFETs.
The physical structure of a MOSFET begins with a slab
of doped semiconductor material called the body.
And for this example, I'll use P-Type Semiconductor material
for the body.
A small section of oppositely doped material,
called a well, is deposited near each end of the body
and in this case those wells are N-Type material.
One of those wells is connected
to a terminal called the source
and the other well is connected
to terminal called the drain.
Between the two wells a layer of insulative material
called silicon dioxide, which is similar to glass
is laid on top of the body.
And then on top of that insulator
is a conductive metal plate connected to a third terminal
called the gate.
These three layers down the center
are what give the Metal-Oxide Semiconductor
Field-Effect Transistor it's name.
To use this type of MOSFET,
I'll connect the source terminal to ground
and connect the drain terminal to a positive voltage source
that will be supplying current through the transistor.
The voltage between the gate and source terminal,
labeled here as VGS,
is what controls whether this transistor
is turned on or off,
whether it is operating in the cutoff region
or the saturation region.
If the source and gate are at the same voltage,
so the difference between them is zero,
this type of transistor will be in the cutoff region.
The P-Type body between the two N-Type wells
puts up a really high resistance
that prevents current from flowing
between the source and drain terminals.
To turn this transistor on, I'll raise the voltage
at the gate until it's above a minium threshold voltage.
Raising the voltage at the gate terminal
means the metal plate attached to the gate
will accumulate more positive charge.
And since oppositely charged particles attract each other
the extra positive charge at the gate
will attract some of the negative charge from the body
causing it to accumulate at the top
near the insulator layer.
This increased amount of negative charge
between the two N-Type wells creates a low resistance path
that allows current to flow from the drain to the source.
Now if I only increase the gate voltage up
to the minimum threshold,
the transistor will only be partially on
and allow some current to pass through.
If I increase the gate voltage to be even higher,
that will attract more negative charges towards the gate
creating an even larger channel
which makes the drain to source resistance even smaller
to allow more current through.
The resistance between the source and the drain
when this MOSFET is completely turned on
is one of the key characteristics of a MOSFET,
labeled here as rDS(ON)
and it's often less than 100 milliohms.
It's important to point out that the insulating layer
of silicon dioxide between the metal gate
and the P-Type body prevents current from flowing in
or out of the gate terminal.
And this highlights one of the key differences
between Bipolar Junction Transistors
and Field-Effect Transistors.
BJTs are current controlled devices.
A small amount of input current to the base terminal
will produce a much larger current
between the emitter and collector.
MOSFETs on the other hand are voltage controlled devices.
The amount of voltage between the source and gate terminal
controls the amount of current
that'll flow through the transistor.
That gives the MOSFET a really high input impedance
at it's gate terminal,
which means it will hardly draw any current
from the device that's generating the control signal.
Now just how Bipolar Junction Transistors
come in two opposite flavors, in P-N and P-N-P
MOSFETs also come in two varieties
that behave in opposite ways, N-Channel and P-Channel.
The N-Channel MOSFET shown here on the left
is the same transistor I demonstrated earlier in this video.
It's called N-Channel, because when a positive voltage
is applied to the gate it attracts
negatively charged electrons to the top of the body,
which create a channel for current to pass through,
between the two N-Type wells.
In the P-Channel MOSFET on the right,
the body is made from an N-Type semiconductor material
and the two wells are both P-Type material.
As it's name implies, the P-Channel transistor
uses positive charge in the body
to create the channel for current to pass through
between the two P-Type wells.
Turning the P channel transistor on
requires the gate terminal to have a negative voltage
relative to the source to attract those positively charged
holes within the body material.
Now, to confuse things just a little bit more,
the two transistors shown here are called
Enhancement Mode MOSFETs.
Enhancement Mode MOSFETs get that name
because when the correct gate to source voltage is applied,
it enhances the channel between the source and drain
which turns the transistor on and allows current to flow.
Enhancement Mode MOSFETs are equivalent
to a normally open switch,
because by default they're turned off and block current.
You have to apply the correct voltage to the them
to turn them on.
Depletion Mode MOSFETs on the other hand
act like a normally closed switch,
because by default they're turn on and let current through.
When a certain gate to source voltage is applied
to the Depletion Mode MOSFET,
it depletes the channels ability to carry current,
which turns the transistor off.
So between these different types of channels and modes,
there are four main types of MOSFETs.
N-Channel Enhancement Mode, P-Channel Enhancement Mode,
N-Channel Depletion Mode, and P-Channel Depletion Mode.
The Enhancement Mode MOSFETs are by far more commonly used
than their Depletion Mode counterparts,
so I'll focus on Enhancement Mode MOSFETs
for the rest of this video.
It's important to be aware
of the different MOSFET types that exist,
to avoid using the wrong one.
When using N and P-Channel MOSFETs in a circuit
the orientation of their source and drain terminal
will be flipped.
On an N-Channel MOSFET, the drain should always be connected
to a higher voltage than the source terminal,
so the voltage drop from the drain to the source, or VDS,
will be positive.
And on a P-Channel MOSFET, the drain voltage
should always be lower than the source,
so the drain to source voltage drop will be negative.
In practice, N-Channel MOSFETs are usually drawn
in circuit diagrams with their source terminal on the bottom
'cause it'll often be tied to ground,
sometimes through a resistor
and for P-Channel MOSFETs, the source terminal
is usually oriented on top,
because it's connected to a positive supply voltage.
In both cases, the drain terminal is typically connected
to the electrical load, that the transistor
will be regulating current through.
In this configuration, to turn the N-Channel MOSFET on,
I need to raise the gate voltage
until the voltage drop from the gate to the source
is greater than a minimum threshold.
To turn on the P-Channel MOSFET, on the other hand,
I'll need to do the opposite,
by lowering the gate voltage until the voltage drop
from gate to the source is more negative than the threshold.
Another way to look at that relationship
between the gate and the source
is that in either type of Enhancement Mode MOSFET,
if the gate and source have roughly the same voltage
the transistor will be turned off.
Raising the gate voltage high enough above the source
will cause an N-Channel MOSFET to turn on
and lowering the gate voltage far enough below the source
will cause the P-Channel MOSFET to turn on.
The schematic symbols used to represent
Enhancement Mode MOSFETs have quite a few variations,
which can get a bit confusing.
The simplest symbol, shown here with three terminals
is commonly used to represent MOSFETs
when they're used in digital circuits.
There's a tiny circle on the P-Channel MOSFET
that indicate that it's control signal
has the opposite polarity of the N-Channel MOSFET.
Another three terminal symbol used to represent MOSFETs
looks similar to the symbols used
for Bipolar Junction Transistors.
In both of these symbols,
the arrow is connected to the source terminal,
similar to how the arrow on a BJT symbol
is connected to the emitter terminal.
And in both cases, the arrow points in the direction
of the conventional current flow through the transistor.
Now, the third set of transistor symbols
is where things start to get confusing,
because it adds an extra terminal to the middle,
representing the body of the transistor.
Behind the scenes, MOSFETs actually have four terminals
and the fourth terminal is connected to the body
or bulk of the transistor.
The reason for having a body terminal
is beyond the scope of this video,
but you usually don't even need to think about it,
because the body will be directly connected
to the source terminal inside of the transistor package.
This four terminal MOSFET symbol indicates that
by connecting the middle body terminal to the source leg.
Additionally, notice that the arrows have moved
from the source terminal to the body terminal,
and more importantly it's flipped directions.
So pay close attention when you're reading
or drawing schematics to make sure you use
the right type of MOSFET.
The broken bar between the source, body, and drain terminals
indicates that this is an Enhancement Mode Transistor,
which behaves like a normally open switch,
hence the breaks.
In the symbol for a Depletion Mode Transistor,
the bar between those three terminals in continuous,
to indicate that it acts like a normally closed switch.
Finally, one more accessory you might encounter
on this symbol is a little upward facing diode
between the source and the drain.
MOSFETs are especially sensitive
to a electrostatic discharge,
so many MOSFET devices will include an internal diode
to protect itself.
Although there are other variations of the MOSFET symbol,
these are the primary ones you'll encounter in the wild.
And if you're looking at older circuit diagrams
or hand drawn schematics, you may also see a circle
drawn around the transistor symbol,
which makes it easier to read and busy drawings.
You can usually find MOSFET components in similar packages
as Bipolar Junction Transistors.
The TO-02 is a popular three terminal package
for low powered MOSFETs that works well
for breadboarding circuits.
And the TO-220 is its counterpart
for high powered transistors
that need to dissipate more heat.