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Why Vt-Channel Length curve differs from Short Channel Effect?

richardyuan

The Vt-L plot looks weird. I cannot see a Vt drop down when L is reduced to near 0.18um for my nmos.

And even change the Vbs, there is no change for the curve.

Can anybody help to explain that?

  • Using this example:

    // generate vt vs l graphs

    model nch bsim4 type=n

    M0 (d g 0 b) nch w=1u l=0.2u
    Vd (d 0) vsource dc=1
    Vg (g 0) vsource dc=1
    Vb (b 0) vsource dc=0

    sweepvbs sweep dev=Vb param=dc start=-3 stop=0 step=1 {
      dc dc dev=M0 param=l start=0.18u stop=10u lin=50
      //dc dc dev=M0 param=l start=0.18 stop=10 lin=50
    }

    save M0:vth

    I get this:

    I suspect the issue is that you are sweeping from 0.18 metres to 10 metres, not 0.18um to 10um. If I do a similar sweep, then I get miniscule variation in the Vt too.

    Regards,

    Andrew.

  • Dear Andrew,

    Thank you very much.

    I checked the component parameter "Length" and found that the unit is meter really. Then I changed the ocean script:

    ......

    analysis('dc ?saveOppoint t ?param "Length" ?start "0.18e-6" ?stop "10e-6" ?lin "50")

    desVar( "Length" 0.18e-6)

    ......

    And finally I got a similiar image like you.

    By the way, your script looks neater and different from ocean skill. What is that and how to use it?(I'm a beginner for spectre simulation indeed)

    Can you introduce the method briefly(or the reference manual I should take a look)?

  • The example I showed was a handwritten spectre netlist, that's all. See the Spectre Reference and User Guides for more details.

    Regards,

    Andrew.

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