Threshold voltage calculation

What do you mean by the "exact threshold voltage"? Most devices output this as an operating point parameter - for example, if you do "spectre -h bsim4" you'll see a section labelled "Operating Point Parameters" and within that you have:

 Operating Point Parameters
==========================

1       region=triode     Estimated operating region. Spectre outputs the number (0-4) in a rawfile. Possible values are
                          off, triode, sat, subth, and breakdown.
...
9       vgb (V)           Gate-bulk voltage.
10      vth (V)           Threshold voltage (alias=lv9).
11      vdsat (V)         Drain-source saturation voltage (alias=lv10).
12      gm (S)            Common-source transconductance (alias=lx7).
13      gds (S)           Common-source output conductance (alias=lx8).
14      gmbs (S)          Body-transconductance (alias=lx9).

So if you do a DC operating point analysis (assuming you're using spectre, you didn't say) in ADE (again, you didn't say if you're using the Analog Design Environment), you can then do a Results->Print->Operating Point and click on the device in the schematic and look at the vth. Or you could annotate the operating point parameters on the schematic and by using the Edit->Component Display in the schematic or View->Annotation... menu (in IC616 onwards) you can choose what gets annotated.

Whether the model's implementation of vth is what you're expecting or not by "exact threshold voltage" is another question!

Regards,

Andrew.

It helps.

how can we check a and Km value when we calculate Vth? Thanks

First of all, please don't post a new question in a 6 year old thread - please see Guidelines for the RF Design Forum

Secondly, what are a and Km? I've no idea what you're referring to.

Andrew.