We went over the architecture and operation of MOSFETs in a previous tutorial but I, personally,
have always struggled with understanding the different operating regions of a MOSFET.
I think it’s mostly due to the terminology but we’re going to take a stab at it today
and remove the confusion about the different regions of operation, their names, and their
applications.
First, a quick review of the MOSFET itself.
Here, you can see the body of an NMOS in enhancement mode - the source and drain, the metal oxide
gate, and the channel.
The first region of operation is the easiest and one that is rarely confused.
In an enhancement mode device, this is when there is no voltage on the gate, there is
absolutely nothing happening in the channel, and no current can flow.
This is called the cut-off region.
We’re not going to spend any more time on this because, frankly, it’s pretty straightforward.
As the gate voltage increases, we enter the second region of operation, the saturation
region.
This is where the depletion region forms beneath the gate and the depletion regions of the
PN junction at the source and drain come together.
You start to collect charge carriers near the drain at this point, but they don’t
span across the entire channel.
If you put a voltage across the drain and source, you may get a current but it won’t
be very large.
Saturation always confused me because in my mind that meant the channel was saturated
with free electrons.
That is *not* the case, however.
It means that the current is saturated and can’t get any bigger.
In saturation, the main factor limiting the current flow is the gate voltage, not the
drain source voltage.
Most applications of MOSFETs use them in this region and if the drain-source voltage is
the same as the gate-source voltage, you can assume you’re in saturation.
Finally, as the gate voltage gets higher than 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 if you put a voltage across the drain and source, you will get a current that is
linearly related to the voltage.
As you increase the drain-source voltage, the current will go up, if you decrease the
voltage, the current will go down.
Because of this, this region of operation is called the ohmic region, or the linear
region, or the triode region.
The terms “ohmic” or “linear” make sense to me because it is acting like a linear
resistor at this point.
The term triode is because the drain current of the FET depends on the drain voltage of
the MOSFET, which is similar to vacuum triodes from back in the day.
I guess.
An important part of this region is that if the drain-source voltage gets too large compared
to the gate voltage, the MOSFET will go into the saturation region.
So, let’s look at these different operating regions from another perspective, using a
very common style of graph you’ll see when dealing with MOSFETs.
On this graph, we have the drain current, ID, on the y-axis and the drain source voltage,
VDS on the x-axis.
Now, if the gate voltage is 0, as the drain-source voltage increases, there will be no current
besides a negligible leakage current.
But, now assuming we have a gate voltage of 1V, as VDS increases, the current will increase
linearly until ID gets too big and the amount of current saturates, so that no matter how
much more you increase VDS, ID will not get any bigger.
If we increase our gate voltage to 2 volts, you can see that you can get a higher VDS
with more current flows before saturating.
Comparing the different gate voltages, you can see that greater gate voltages yield a
larger “ohmic” or “triode” region before ID saturates.
Another part that confused me about this is that you can change regions not only by changing
the gate voltage but also the drain-source voltage.
If you think about the regions only in regards to the gate voltage, that’s an easy, yet
incomplete view of how it works.
And to be explicit, when we talked about the regions of operation before, we looked at
the inversion channel being created as the gate voltage, so, as we increase the gate
voltage, we’re passing from cut-off, saturation, and finally to the triode region.
But, if you look at this graph, moving from left to right, we pass from the triode region
to the saturation region, and that’s because we’re not increasing the gate voltage, we’re
increasing the drain to source voltage while assuming a set gate voltage.
So be cautious when trying to compare this graph with your intuitive understanding of
what’s happening at the semiconductor level of the MOSFET while switching regions of operation.
Finally, just to give a quick comparison to something we’re more familiar with, a MOSFET
acts like a water spigot.
The gate voltage is opening the tap, while the drain-source voltage is the water pressure.
As you open the tap, no water will flow until there is water pressure.
And, if the tap is open and the water pressure increases, it will increase linearly for awhile,
in the linear mode, until no more water is capable of leaving the tap, at which point
it will be saturated unless the tap is somehow opened even more.
And that’s it - we went over the different regions of operation while looking at how
the devices operate at the semiconductor level and also how this translates to how the MOSFET
will work in a circuit with different input voltages.
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