Ive been a bit wondering about calculator function of "operation region". Is is calculated simply by Vgs, Vth, Vds? Or Is there any other thing done for this background?
What calculator function? There's not a calculator function called "operator region".
Perhaps you're asking about how operating point parameter "region" (see "spectre -h bsim4", say) is computed? If so, it's sort of described in the Virtuoso Simulator Components and Device Models Reference manual <MMSIMinstDir>/doc/spectremod/spectremod.pdf
2 region=triode Estimated operating region. Spectre outputs number (0-4) in arawfile.Possible values are off, triode, sat, subth, or breakdown.
Thats what they say in spectremod.pdf but it does not mean how each region was defined. So my question is, say, when Vgs>Vth and Vds>Vdsat, then "region" would be simply saturation? Or Is there more complicated computation?
Try these two solutions, one for bjt and one for bsim3v3 (I think it would have the same meaning for bsim4).
You can map the 5 regions as follow:
3 subth and 4 breakdown:
Condition for saturation region in zeroth approximation is: