How to set Vth (voltage threshold) for MOSFET?

This is normally set as a model parameter in your transistor models - it's often called something like vto (e.g. in "spectre -h mos1" you'll see this). This is a slightly odd request - normally a user wouldn't set the threshold voltage themselves as it is part of the model characterization process.

That said, you didn't say what simulator you're using?

Andrew.

 Andrew

 This is exactly a dilemma I got now.  I am looking for n elegant way to mimic mismatch (for example
specific MOSFET Vth).

I use pectre. Does the "alter"  command allow me to tweak specific device Vth ?

I know how to modift the MODEL Vth, but it will be common to all devices .

I want to avoid adding a parametric Vdc to the device gate, to keep the original netlist.

Best Regards

Eliav

Eliav,

The issue is that you need a unique model for each instance - which is something that monte carlo does for you. Can you not just use monte carlo? You can't alter a model parameter for a specific "dev" - because the models are shared for efficiency reasons.

Regards,

Andrew.

 Andrew

 I can use monte-carlo, but sometimes it is taking very long time to get the sense of where are the "weak-spots".I am aware to the sensitity tableI can generate afterwords.

Sometimes, the "manual" variation approach yield much faster understanding.Traditionally, it involves adding Vdc to the gate of device of interest. I was looking for a way to achieve this without modifying the netlist OR the PDK.

 I understand that a "model" based mosfet cannot be "alter"ed. But,perhaps if I use a "*_macro" model (which is a subcircuit ) it is possible ? Maybe if I create my own version of the marco subcircuit having a parameter depended  Vdc inside ?

Regards

Eliav

 Hi Eliav,

Actually that's what I was thinking of in the meantime - I think this would be a pretty straightforward way of changing it without disturbing the existing netlist.

Regards,

Andrew.

 Andrew

I got it working well. Thanks

If you don't mind, just another question:

Does it accept wildcard (same rules as save commnd) ?

 This is important since I try to do it on extracted netlist .

Regards

Eliav

Hi Eliav,

No, I don't think you can use wildcards with alter. Sorry!

Andrew.

Hi.. Am a research scholar...in cadence different library models are there like SS, TT,FF where different parameters are used in each model files... In SS model the threshold voltage used is 0.26v, in TT threshold voltage used is 0.36v and in FF threshold voltage used is 0.46v....my question is how these voltages are used...why cant we use 0.3 instead of 0.26 in SS library and 0.4 instead of 0.46 in FF.....is there any reason while selecting these voltages....kindly clear my doubt