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. If you liked this video, or you found it interesting,
please subscribe to our channel, go to circuitbread.com and we’ll catch you in the next one.
