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I have some questions regarding NMOS/PMOS characterization. I'm designing a common source amplifier using gm/Id method and since it requires characterization, I did so but using minimum lengths and widths for NMOS and PMOS (90 nm for all lengths and widths). My question is, is this okay? Are there no restrictions to lengths or widths when characterization? Also, an assumption was mentioned here that width of PMOS is thrice of NMOS. Is this applicable all the time and should this be followed always? Because my final width and length values do not follow this trend and yet it can achieve the desired specs.

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  • \$\begingroup\$ It depends :) It always depends on what your goal is. I am unsure what do you mean characterisation. 90nm is a fairly mature node. Your transistor models should be accurate enough. Try to be a little more specific. What do you want to do? What is your goal? How would you achieve it? Where do you see the problem you are asking about? \$\endgroup\$ Commented Nov 2, 2018 at 16:14
  • \$\begingroup\$ By characterization, I meant generating plots for Id vs vgs, gm/Id vs vgs etc. by using a test transistor with test widths and lengths. This seems to be the first step when designing using gm/Id. As to being specific, I'm just concerned as to the 90nm I used as length and width since examples of other designs I've seen using gm/Id uses a width that is much larger than length but didn't mention if this was significant or not. Also, my design is trying to achieve a certain gain and bandwidth, for a start. \$\endgroup\$
    – user203020
    Commented Nov 2, 2018 at 16:21

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Always use the same transistor width and length what you would like to use in your circuit. If you do not know yet, start with your best guess.

Using minimum length will reduce your threshold voltage. But also would a minimum width on an STI process. Once you have a fairly high channel length, it will not vary any more. Unless you design an RF circuit, it is better to use longer channels than the minimum to reduce noise and mismatch.

Layout dependent effects could have an impact on your design, but I guess by default the model assumes that you have enough dummies. So I would not worry about that.

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