Question about MOSFET Saturation region

In summary: I have checked the current definitions on the net, and I have seen that they have introduced "saturation" as a description of the "pentode" region of the MOSFET. However, it is very different from the saturation phenomenon in bipolar transistors. A bipolar transistor in saturation has a VCE that is independent of the base current, a MOSFET in "saturation" has an IDS that is independent of VDS.
  • #1
Dextrine
102
7
Ok, so I've read around and have become quite confused with, not only the terminology about whether a device is in linear or saturation or ohmic or active region, but now my whole concept of mosfets is on the brink of breaking down!

This is my understanding.

mosfets.JPG


In the attached picture, the maximum current that can flow is 1A. The higher Vgs, the higher the POSSIBLE current (if the load were smaller for example). Therefore, if I have a high Vgs, then the current through the mosfets will vary as a function of my input voltage which is NOT what I want??

I know I need the resistance across the mosfets to be as small as possible to minimize conduction losses, it's just confusing where exactly that happens. I thought the Rdson is the smallest, the mosfet is fully on, in the saturated (active) region.

However, this makes less and less sense to me as time goes on. If someone could give any definitive answer as to how the mosfets in my example should be biased to give the lowest conduction loss.

Thanks for any help.
 
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  • #2
  1. MOSFETs do not go into saturation.
  2. MOSFETs act a voltage-controlled resistors. Look for RDS(on) in the specification sheet. You will get a set of values, corresponding to different values of VGS
 
  • #3
Ok, I think I figured it out. If you want to use a MOSFET as a switch, you must have VGS be as high as possible in order to keep VDS as low as possible and thus minimize conduction losses. This is called the triode, or ohmic, or linear region.

Operating the device as a voltage controlled current source is NOT what you want if you want to use it as a switch. This is called the active, or saturation region.

Can anyone confirm this?

and also, Svein, I'm not sure what you mean by mosfets do not go into saturation, there is always a clearly marked saturation region.
 
  • #4
Svein said:
  1. MOSFETs do not go into saturation.
MOSFETs certainly do go into saturation. I think the thing that is confusing you is that the terminology is confusing. Look at the two attached images from Wikipedia. While they look similar, the region on the right of the MOSFET curve is called saturation, while the region on the left of the BJT curve is called saturation.
 

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  • #5
phyzguy said:
MOSFETs certainly do go into saturation. I think the thing that is confusing you is that the terminology is confusing. Look at the two attached images from Wikipedia. While they look similar, the region on the right of the MOSFET curve is called saturation, while the region on the left of the BJT curve is called saturation.
So would you say my second post has me understanding the subject correctly?
 
  • #6
Dextrine said:
So would you say my second post has me understanding the subject correctly?

Yes, I think you have summed it up well.
 
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  • #7
Dextrine said:
Svein, I'm not sure what you mean by mosfets do not go into saturation,
I have checked the current definitions on the net, and I have seen that they have introduced "saturation" as a description of the "pentode" region of the MOSFET. However, it is very different from the saturation phenomenon in bipolar transistors. A bipolar transistor in saturation has a VCE that is independent of the base current, a MOSFET in "saturation" has an IDS that is independent of VDS.

A "rule of thumb" might be: A bipolar transistor in saturation acts as a constant voltage generator, a MOSFET in "saturation" acts as a constant current generator.
 
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  • #8
Svein said:
I have checked the current definitions on the net, and I have seen that they have introduced "saturation" as a description of the "pentode" region of the MOSFET. However, it is very different from the saturation phenomenon in bipolar transistors. A bipolar transistor in saturation has a VCE that is independent of the base current, a MOSFET in "saturation" has an IDS that is independent of VDS.

A "rule of thumb" might be: A bipolar transistor in saturation acts as a constant voltage generator, a MOSFET in "saturation" acts as a constant current generator.

Agreed. That's why I said the terminology is confusing.
 

FAQ: Question about MOSFET Saturation region

What is the MOSFET saturation region?

The MOSFET saturation region is a region in the MOSFET's operation where the drain current remains relatively constant despite changes in the drain-source voltage. This occurs when the MOSFET is fully turned on and the drain-source voltage is high enough to create a channel between the source and drain.

How is the MOSFET saturation region different from the linear region?

In the linear region, the drain current varies linearly with changes in the drain-source voltage. This occurs when the MOSFET is partially turned on and the drain-source voltage is not high enough to create a channel. In the saturation region, the drain current remains relatively constant, regardless of changes in the drain-source voltage.

What factors affect the MOSFET saturation region?

The main factors that affect the MOSFET saturation region are the gate-source voltage, the channel length, and the drain-source voltage. The gate-source voltage controls the width of the channel, while the channel length and drain-source voltage determine the strength of the channel.

How do I calculate the drain current in the MOSFET saturation region?

The drain current in the MOSFET saturation region can be calculated using the following formula:
ID,sat = 0.5 * kn * (W/L) * (VGS - VT)2
Where kn is the MOSFET's transconductance parameter, W/L is the ratio of the channel width to length, VGS is the gate-source voltage, and VT is the threshold voltage.

Can the MOSFET saturation region be used in circuit designs?

Yes, the MOSFET saturation region is commonly used in circuit designs, particularly for amplification purposes. In this region, the MOSFET acts as a voltage-controlled current source, making it useful for applications such as switching and amplification. However, it is important to consider the limitations of this region, such as the maximum drain current and power dissipation, when designing circuits.

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