Acceptable difference in range of Vds and Vsat to be in subthreshold region

[roki]

Hi,

From what i know,subthreshold region(or weak inversion) happens when gate voltage is less than threshold voltage.
While from the simulation diagnosis,i found one of my NMOS to be in linear region,what is the acceptable difference of Vdsat and Vds of a MOSFET for to be consider in subthreshold region. My lecturer told me that difference of 0.2V is pretty acceptable for it to be in subthreshold. Should lesser than 0.2V be better?

 

[erikl]

Re: .. difference in range of Vds and Vsat to work in saturation region, in weak inv.

I think your question isn't quite clear, may be you mix up subthreshold and saturation resp. linear region?

Subthreshold (or moderate resp. weak inversion) is an operation mode, meaning in which range of Veff = VGS - Vth the FET is operated, whereas the linear or saturation region designate in which part of the ID vs. VDS characteristic it is operated:

VDS < VDS,sat : linear mode
VDS > VDS,sat : saturation mode

So your question IMHO should be changed to
"what is the acceptable difference of Vdsat and Vds of a MOSFET for to be considered in saturation, when working in subthreshold mode?"

See here a log(ID) vs. VDS plot with a dot-marked VDS,sat curve:

Left of the dot-marked VDS,sat curve is the linear region, right of it the saturation region.

At the right side, the inversion modes are marked: S.I. for strong, M.I. for moderate, and W.I. for weak inversion mode. As you can see, in weak inversion mode VDS,sat = 4*Vt ≈ 100mV. So in W.I. mode, with a VDS=200mV (i.e. with a difference of just +100mV to VDS,sat) you can be sure to operate the FET in saturation mode.

 

[roki]

Re: .. difference in range of Vds and Vsat to work in saturation region, in weak inv.

So your question IMHO should be changed to
"what is the acceptable difference of Vdsat and Vds of a MOSFET for to be considered in saturation, when working in subthreshold mode?"

See here a log(ID) vs. VDS plot with a dot-marked VDS,sat curve:
View attachment 76001
Left of the dot-marked VDS,sat curve is the linear region, right of it the saturation region.

Hi erikl, i'm with you.The question is indeed more well structured.
And your explanation is easy to understand.
Thank you!!