Let’s talk about FETs or, specifically, MOSFETs.
We’ve done videos about bipolar junction transistors, or BJTs, and while MOSFETs may
also be transistors and share many similar properties superficially, the way they operate
is completely different.
MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.
Right now, that may just seem like random words mashed together but once you understand
how it works, this name will make perfect sense.
MOSFETs still act like switches, by varying the voltage on one terminal, the gate, it
changes the resistance between the other two terminals, the source and drain.
Let’s discuss the makeup of a MOSFET, using an NMOS in enhancement mode as an example.
With an NMOS, you have a p-type substrate that you then create two heavily doped n-type
regions.
These two regions are called the source and the drain region.
With our knowledge of semiconductors, you can see that you’re creating a PN junction
between the substrate and these two regions.
On top of the substrate, an oxide, which acts as an insulator, is deposited.
Then, on top of that, a layer of metal is deposited, which finalizes the gate structure.
Now you can see where the Metal Oxide Semiconductor in MOSFET comes from.
But it may seem strange to have your gate being completely electrically isolated from
the rest of the circuit.
This is where the FET term comes in.
Even though there isn’t a direct electrical connection, the voltage on the gate creates
a field effect.
As we know from our studies about diodes, at a PN junction, a depletion region is naturally
created even when there are no electric fields.
This is the natural state when the gate voltage is 0, and the MOSFET is operating in the “cutoff
region”.
This is an operating region, not a physical region, which can be confusing.
If you increase the gate voltage, that positive voltage will repel holes in the substrate
away from the area between the source and drain, an area called the channel region,
this time a physical region.
As the free holes leave the channel, only negative fixed ions are left, creating a depletion
region across the entire channel.
Besides this depletion region, an inversion layer of electrons starts to form at the source
and, as the voltage increases, that inversion layer expands toward the drain.
However, at this point, when the gate threshold is not yet equal to the threshold voltage,
free carriers do not yet connect all the way from the source to the drain.
This area of operation is called the saturation or pinch-off region.
However, as the gate voltage continues to increase, increasing the electric field, and
finally passes the threshold voltage, electrons from the source and drain flow in and form
an inversion layer of electrons that connect the source and drain regions.
Now that they’re connected, if a voltage is applied across the source and drain, a
current will flow.
The MOSFET is now operating in the linear or triode region.
So this is how, using a metal oxide semiconductor structure, you can use the field effect of
a gate voltage to create a semiconductor switch.
And to get a PMOS device, you have an n-type substrate and heavily p doped wells for your
source and drain, and the current carriers are holes instead of electrons.
Also, I mentioned earlier that this is an example of an enhancement mode device.
It’s called an enhancement mode device because an increased gate voltage “enhances” the
conductivity of the channel.
Some MOSFET’s are designed so that they naturally have a conductive channel and a
negative gate voltage is needed to actively turn it “off”, and these are called depletion-mode
devices.
Conceptually, that’s basically all you need to know to understand the mechanics behind
MOSFETs, for the most part, everything stems from those operating principles.
But there are a few things that I’d be remiss to not mention that can affect their operation.
First is the channel length, L - the distance between the source and drain.
Second is the channel width, W - which is how long the source and drain are.
These two features are very important when it comes to designing a MOSFET.
Third, in this example, we assumed that the substrate, or base, was connected to ground.
That’s usually the case but not always.
No matter what, you need to make sure that the source and drain are at equal or higher
voltage potentials than the substrate or else you will forward bias that PN junction and
get an unwanted current.
This topic can be surprisingly confusing and I personally believe that it’s in large
part due to the amount of terms, especially those that stand for the same thing, such
as linear, triode, and ohmic to describe the same region.
Unfortunately, that’s just the reality of the situation and it will just get easier
with experience and familiarity.
I hope that this at least gives you a good foundation as you start to use or design MOSFET
transistors, so you have an intuitive understanding of how these operate.
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and we’ll catch you in the next one.