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For someone trying to learn about FETs, it seems there is no consistency in terminology (unless I am reading things wrong.)

I am trying to understand the FET as a switch (for power applications.) It has become clear that you want to operate a switch in the ohmic region. To get to the ohmic region you would need to go from cut off -> saturation -> ohmic region.

TI refers to it like this in an app note:

Snippet from a TI application note

From this I implied that the 'linear' region is in fact rather confusingly the saturation region of the FET as shown all across the web for a FET curve:

enter image description here

However, when I went to wikipedia I came across the following image:

enter image description here

This is saying the exact opposite of what I understood which is saying that the ohmic region is the linear region.

Infineon has an app note around FET switching, titled Linear Mode Operation

They refer to the linear mode as the saturation region as shown from this:

enter image description here

So what is it then?

Also just to be clear: Is the reason we want to minimise time in the saturation region with an FET because here we have a VDS and an ID current flowing through the FET, whilst as soon as the FEY reaches linear region, it's VDS would fall close to 0 and hence minimise losses?

When we say the FET is saturated, it means for a given Vgs, Vds makes no difference to drain current (because channel can't let more electrons to flow?) Shouldn't the ohmic region be called saturated? Since increasing VGS makes no difference to drain current as the ID is now limited by the circuit and not the FET?

Edit: Answers in electronic stack exchange say it is the saturation region:

Link1

link2

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  • \$\begingroup\$ #Hasman404, When I first read the second paragraph of your question, my impression was that you have mixed up (messed up :)) all the terms/concepts! The root cause of your confusion/inconsistency is that you have not differentiated between linear mode and linear region. So you won't appreciate that for a MOSFET, you can operate in linear mode in the saturation region. This picture hopefully helps: i.imgur.com/CG5leI1.jpg. Happy thinking! Cheers. \$\endgroup\$ – tlfong01 Feb 27 at 13:56
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    \$\begingroup\$ Why is the saturation region being called linear mode? When there is nothing linear about it? More so when there is already the linear region (ohmic region where the current linearly increases with VDS). Additionally it seems TI uses the word linear region when they are referring to saturation region, so there is discrepancy there. \$\endgroup\$ – Hasman404 Feb 27 at 16:45
  • \$\begingroup\$ #Hasman404, (1) Ah, as I said, the crux of the matter is the difference between region and mode. You might like to google the meanings of "region" and "mode" to clear your little confusing head. (Yes, I did google that.) (2) Your referred TI paper does not seem to include a picture showing the saturation region and linear mode. Please show me the TI picture. (3) I googled a NS paper explaining the confusing terms and also point out that the terms "saturated" and "linear" in BJT is opposite in MOSFET, causing huge confusion. / to continue, ... \$\endgroup\$ – tlfong01 Feb 28 at 4:33
  • \$\begingroup\$ You might like to google the NS paper showing this picture: (4) Explaining MOSFET saturated region and liner transfer characteristic (operation mode) - NS. i.imgur.com/ETWKOdZ.jpg. Happy reading. Cheers \$\endgroup\$ – tlfong01 Feb 28 at 4:36
  • \$\begingroup\$ (5) You asked the following question: Why is the saturation region being called linear mode, when there is nothing linear about it? It would be nice if you can, hopefully, after googling meaning of region and mode, and read the NS paper, have the eureka, and give the answer yourself. But it case you are still got stuck in a blind spot, I am happy to give more hints. Happy critical thinking. Cheers. \$\endgroup\$ – tlfong01 Feb 28 at 4:59
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So what is it then?

This is the correct graph: -

enter image description here

Taken from this wiki page.

Saturation refers to the channel being saturated and, as you said, no matter what \$V_{DS}\$ you apply, current remains constant. It is sometimes also referred to as the active region (not to be confused with the MOSFET being activated or ON).

The linear (triode) or ohmic region is when the MOSFET is used as a switch (ON). Your top graph (in the question) is basically wrong because it doesn't correctly show the different slopes in the ohmic region when you apply different gate voltages. This region is called linear because there are different slopes that are governed by the gate voltage and means the MOSFET can act like a variable resistor hence, it gets the name linear but, different texts use this term rather loosely.

Also just to be clear is the reason we want to minimise time in the saturation region as a FET because here we have a VDS and an ID current flowing through the FET? Whilst as soon as the fet reaches linear region, it's VDS would fall close to 0 and hence minimise losses?

Yes, we want to minimize time in the saturation region because that is when the MOSFET is dissipating the most power and potentially operating below it's ZTC (zero temperature coefficient) and may suffer rapid thermal runaway.

Shouldn't the ohmic region be called saturated?

No, because in the ohmic region the channel isn't saturated. However, for a BJT, that equivalent part of the characteristic is called the saturation region but, for different reasons; for a BJT, it is the base that becomes saturated. Same name, different mechanism, different part of the characteristic.

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  • \$\begingroup\$ The two links i have provided refer to saturation as the linear region. Also is TI in the snippet and link to the app note not referring to the linear region as the saturation region when they say 'it must go through it's linear region'. \$\endgroup\$ – Hasman404 Feb 27 at 11:26
  • \$\begingroup\$ The so-called "linear" region does tend to be misused in some texts. And, I think that's because in a BJT, the saturation is sometimes referred to as the linear region. People forget that BJTs and MOSFETs (despite having similar looking characteristics) have a different saturation mechanism. \$\endgroup\$ – Andy aka Feb 27 at 11:30
  • \$\begingroup\$ That is really annoying.. Infineon has a FET application note called linear operating mode (infineon.com/dgdl/…) They also refer to saturation as the linear region! \$\endgroup\$ – Hasman404 Feb 27 at 11:36
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    \$\begingroup\$ Many share @Hasman404 's confusion. I think it stems from the simple fact that (on your graph) the linear region is populated with curved lines (non-linear), whilst the saturation region is populated with straight lines (linear). It's just exacerbating the confusion... \$\endgroup\$ – Paul Uszak Feb 27 at 12:37
  • \$\begingroup\$ @PaulUszak those straight lines you mention are the most non-linear part of the MOSFET characteristic. The curved lines (despite them being curved) are more linear in terms of ohms law. \$\endgroup\$ – Andy aka Feb 27 at 12:57
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Your answer is approximation.

The following is a graph of output characteristics for an arbitrary 2N7000 MOSFET:-

graph

Assume a fixed gate drive \$ V_{GS} \$ of 5V and ignore all the other curves. So it's fixed. Consider the MOSFET then as an opaque component with only two legs, Drain and Source.

As \$V_{DS}\$ (voltage across component) changes, \$I_D\$ (current through component) changes. And they change almost linearly if you squint sideways at my graph (purple line). Like a resistor. The first part increases linearly, and then goes flat at saturation when it no longer behaves as a resistor. These are approximations, but form the basis of the terminology in your question. The difference is simply real world characteristics being more complex than theoretical approximations. And confusing graphs.


Actually the initial slope is \$ \propto V_{GS} - V_T \$ where \$V_T\$ is the threshold voltage.

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