MOSFET operation in subthreshold region

Hi All,
Is applying the small-signal model for the MOSFET transistor that operates in subthreshold region correct?
Are “gm” and “rO" values or equations in subthreshold region as saturation region???????
Please help me…

Are “gm” and “rO" values or equations in subthreshold region as saturation region???????

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Hi Ania
A comment on ro . when your current has changes , ro will be changed . so if your required out put resistance is high , it is important to pay more attention to the ro .
By the way , what do you mean by subthreshold region ? are ou referring to the linear region ?
Best Wishes
Goldsmith

goldsmith said:

Hi Ania
A comment on ro . when your current has changes , ro will be changed . so if your required out put resistance is high , it is important to pay more attention to the ro .
By the way , what do you mean by subthreshold region ? are ou referring to the linear region ?
Best Wishes
Goldsmith

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I have been through the learning process with mosfet and subthreshold and I believe it is as follows.

You need to apply at least 10V to the gate of a fet to get it to fully open up and offer little resistance to large currents.

If you apply less than 10V then the fet is said to operate in the ohmic region where it is not fully open and offers increased resistance to source-drain current flow.

I believe this puts increased stress on the fet and shortens its lifespan.

A number of people have recommended to me that you apply your logic signal to a purpose designed mosfet gate driver IC and have it operate the mosfet gate rather than doing so directly from the logic signal or from the various discrete transistor drivers that people come up with.

If you google "mosfet gate driver IC" you are bound to find dozens of such ICs - there are some available on ebay.

Are “gm” and “rO" values or equations in subthreshold region as saturation region???????

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Ania! Subthreshold region is where Vgs is less than threshold voltage and still it starts conduction.
"ro" specifically named in saturation region only, and gm value becomes very small in sub-threshold region and it follows exponential equation as bipolar devices do.
So subthreshold region has very less gain and no "r0" term.

Operation in the "subthreshold region", which is below the normal threshold voltage, is often used in low power analog circuits since the operating current is very low and the gain is high (see "Modes of Operation" under this for further info). gm and r0 still apply in the subthreshold region but their values are different from the saturation or active region.

crutschow said:

Operation in the "subthreshold region", which is below the normal threshold voltage, is often used in low power analog circuits since the operating current is very low and the gain is high (see "Modes of Operation" under this for further info). gm and r0 still apply in the subthreshold region but their values are different from the saturation or active region.

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Can you do that with any mosfet or are there specific types of mosfets designed for that purpose?

Could you operate a power mosfet in that manor for example without damaging it?

boylesg said:

Can you do that with any mosfet or are there specific types of mosfets designed for that purpose?

Could you operate a power mosfet in that manor for example without damaging it?

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Any MOSFET will work in the subthreshold region. But why would you want to with a power MOSFET since it's only useful for amplifying very small signals at very low transistor currents. :-? It certainly wouldn't damage it.

in general the mosfet in sub threshold region behaves like a linear device voltage across the drain and source varies linearly with that of the drain current

and primarily this region is not preferred as consistency is very low in these regions

low power ip can be operated but the operating point is hard to be fixed in these areas