How to perform a MOSFET area sweep while maintaining the aspect ratio, DC analysis

Dear  mukulmishra5,

Dear mukulmishra5,

mukulmishra5 said:

When I run the test case with just "FET_len" assigned for Length and a numeric value for the Width, the simulation works fine but when I use variable for width it fails and gives below error:  

I think you need to use the proper syntax for a design variable. Did you try entering:

VAR("FET_len") as the entry in "Length (drawn)(um)" entry box in lieu of only FET_len?

mukulmishra5 said:

Question 1: Is this the correct way of solving the problem at hand?

I do not know what technology you are using, but many technologies only allow specific channel lengths and widths. If so, you may not be able to realize a constant aspect ratio. Specifically, suppose the allowable channel lengths are Len_i (i = 1,2,3...) and the allowable widths are W_j (j = 1,2,3...). It may be that

3*FET_len = 3*Len_i results in a value for W that is not included in the allowable set of W_j.

Hence, you may need to limit your choice of values for Len_i to those that when multiplied by your desired aspect ratio produce a value of W in the set of W_j. I hope my explanation makes sense to you mukulmishra5! It may not be relevant for your technology, but withou knowing your process, I thought I should at least mention it.

Shawn

ShawnLogan said:

I think you need to use the proper syntax for a design variable. Did you try entering:

VAR("FET_len") as the entry in "Length (drawn)(um)" entry box in lieu of only FET_len?

This suggestion is incorrect. You do not use the VAR() syntax on the properties on an instance - it's used for design variables/global variables and output expressions in ADE but not on the create instance/edit properties form (it's not a valid "Analog Expression Language" function). There you use the variable name directly (which also generally works in design variables/global too).

Shawn's point about this might be producing an out-of-range width is valid (and it is indeed hard to know without knowing the technology) but some PDKs (quite a lot of PDKs) do not work correctly when you parameterise device values this way as they require a callback to be executed to compute some derived parameters. That cannot work if you've put a variable name on the device itself.

Instead, you can use ADE's parameterisation capability. This is available in ADE Explorer and Assembler (and the older ADE XL), and is in the "Parameters" section of the ADE UI. It allows you to override a parameter value on a specific instance and add a sweep or set it to a specific value or variable name (which can then be swept too). The benefit of this approach is that the schematic itself does not need to be edited, plus the callbacks get invoked for each sweep value which won't happen if you've just put a variable directly on the instance.

Note that this approach won't help if the value is out of range, but at least it's worth exploring.

Andrew

Dear Andrew and mukulmishra5,

Andrew Beckett said:

This suggestion is incorrect. You do no use the VAR() syntax on the properties on an instance - it's used for design variables/global variables and output expressions in ADE but not on the create instance/edit properties form

Thank you, Andrew, for both reading my post and correcting my response - and  I am sorry mukulmishra.  I hope you did not "waste" any time trying my incorrect suggestion!

My apologies...sigh...

Shawn