Not sure what analysis you're doing, but if you want to plot the Vth from anything other than a DC operating point, you'll need to get the simulator to save the vth. You can do this with:
where M1 is the name of the transistor. This would need to go into the spectre netlist - if you're using spectre from the Analog Design Environment, you can create a file called (say) "mysave.scs" with the above contents, and then include it by referencing it from Setup->Model Libraries.
Having done that, you can access the vth for whichever analysis you're using by using the Results Browser in ADE.
In reply to Andrew Beckett:
I am doing transient analysis for the rectifier for very low power applications.
I want to lower the Vth of the MOS and so I want to know Vth for different aspect ratio's.
I will try the method suggested by you once I am in lab.
If no success, I will post further details.
Thanks & regards,
In reply to Shahnaf:
I tried the method suggested by you. I have used four MOSes in my circuit.
So, I included all four MOSes in .scs file to save the Vth.
Here are some of the observations and queries:
1. When I do Transient analysis, I opened the results browser -> tran-tran . I found only two MOSes in that list. But when I click Vth, it plotted Vth vs time.
2. I want to plot between Vth and W or L or Vth and aspect ratio (W/L) and so I chose ac analysis by sweeping variable 'w' (width of Tr). To my great surpise , I found none of the transistor list in Results Browser -> ac-ac.
Do I need to do anthing more in order to plot Vth vs W, Vth vs L and Vth vs W/L?
If so, what should be done?
Thanks in advance.
In response to your points: