# FET linear and saturation regions

#### n9352527

Joined Oct 14, 2005
1,198
Quoting from Volume III, Chapter 5, Part 4 (link):

"...the saturation (non-horizontal) portion of a JFET's characteristic curve is sometimes referred to as the ohmic region."

Equating saturation portion of the FET characteristic curve with ohmic region would be confusing, if not inaccurate, since most textbooks actually refer to the region where the drain current nearly independent of the drain source voltage (the horizontal portion of the curve) as saturated region and the ohmic region as non-saturation/linear/triode region. The opposite!

In saturation region, the drain current is nearly independent of the drain-source voltage (hence the notion of saturation) and the relationship of VDS > VGS - VTH holds true.

In linear or ohmic region, the drain current does depend on, and varies linearly with, the drain-source voltage and the relationship of VDS < VGS - VTH is true.

I realise that this naming convention is confusing and counter-intuitive, especially because when a FET switch is fully on (where the drain-source voltage is saturated w.r.t drain current) we call the FET switch in saturation, although it is actually operating in linear/non-saturation/triode/ohmic region.

Also, the fact that the naming conventions of equivalent regions in BJT characteristic curve is the opposite (ohmic -> saturated, saturated -> active) doesn't help either!

#### Dcrunkilton

Joined Jul 31, 2004
422
Originally posted by n9352527@Apr 5 2006, 11:49 AM
Quoting from Volume III, Chapter 5, Part 4 (link):

"...the saturation (non-horizontal) portion of a JFET's characteristic curve is sometimes referred to as the ohmic region."

Equating saturation portion of the FET characteristic curve with ohmic region would be confusing, if not inaccurate, since most textbooks actually refer to the region where the drain current nearly independent of the drain source voltage (the horizontal portion of the curve) as saturated region and the ohmic region as non-saturation/linear/triode region. The opposite!

In saturation region, the drain current is nearly independent of the drain-source voltage (hence the notion of saturation) and the relationship of VDS > VGS - VTH holds true.

In linear or ohmic region, the drain current does depend on, and varies linearly with, the drain-source voltage and the relationship of VDS < VGS - VTH is true.

I realise that this naming convention is confusing and counter-intuitive, especially because when a FET switch is fully on (where the drain-source voltage is saturated w.r.t drain current) we call the FET switch in saturation, although it is actually operating in linear/non-saturation/triode/ohmic region.

Also, the fact that the naming conventions of equivalent regions in BJT characteristic curve is the opposite (ohmic -> saturated, saturated -> active) doesn't help either!
[post=15857]Quoted post[/post]​

You appear to have found an error. What you say makes sense. I will try to verify this against one other source-- say a text book. In the likely event that this proves true, I will list you as a contributor in the contributors Appendix-2. I am out of town this week. So, It will be a week or so before I have access to my books. At that time I will post back to this thread and make the correction

Thanks for the input

#### Dcrunkilton

Joined Jul 31, 2004
422
Originally posted by Dcrunkilton@Apr 13 2006, 06:57 PM
You appear to have found an error. What you say makes sense. I will try to verify this against one other source-- say a text book. In the likely event that this proves true, I will list you as a contributor in the contributors Appendix-2. I am out of town this week. So, It will be a week or so before I have access to my books. At that time I will post back to this thread and make the correction

Thanks for the input
[post=16154]Quoted post[/post]​
As of this date, I can verify that everything you say is true according to J. Watson, "An Introduction to FETS", Compiled for Siliconix, 1970. I will post back here once I have made changes to the copy at ibiblio.

#### Dcrunkilton

Joined Jul 31, 2004
422
Originally posted by Dcrunkilton@Apr 30 2006, 12:46 AM
As of this date, I can verify that everything you say is true according to J. Watson, "An Introduction to FETS", Compiled for Siliconix, 1970. I will post back here once I have made changes to the copy at ibiblio.
[post=16698]Quoted post[/post]​

The changes have been made to the master copy of text at ibiblio near this Figure. Changes have been made to the text and the Figure to conform to standard industry terminology. Hopefully this will un-confuse our section on FETS\s.

Thanks for the input