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how to determine the subthreshold process paramter?

MenghanSun

Dear all,

I have a question that i couldn't find an answer to in this forum. I wish to find out the subthreshold process paramter (sometimes referred to as process slope, subthreshold slope) of a given MOS device.

Id = u*Cox*(Vt^2)*(W/L)*exp( (VGS-Vth) / m*Vt ) * [1 - exp(-VDS/VT)]

The "m" is the subtreshold slope. I ran the DC simulation, use OPT on a MOSFET, I can find a big list of device parameters, region, self-gain, betaeff, and etc. However, I can't find the "m" the subthreshold slope.

Could anyone tell me how to find out the subtreshold slope "m" in cadence, Any help is appreciated. Thank you!

Menghan

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  • MenghanSun

    Hi Andrew,

    Thanks for the reply. I uses spectre mmsim14, virtuoso 616

    This is not a particular model. This is a general fact. For a reasonable channel length (Lmin => 500nm), in saturation, strong inversion, the mosfet exhibits square law characteristics. In subthreshold(weak inversion), the mosfet exhibits exponential characteristics like a BJT, with the addition of "nonideality factor" or "slope factor".

    If we only look at the the exponential term:

    BJT: exp{ [Vgs-VT] / [kT/q]  }

    MOSFET: exp{ [Vgs-VT] / [n*(kT/q)]  }

    This additional n term is typically around 1~1.5

    In the slide 15 in the below link, n is defined as:

    n = 1 + Cdep/Cox

    Which seems like something that would vary with operating point. Maybe I could try to Cdep and Cox in the list of operating point parameter??

     Elad Alon's notes below:

    http://bwrcs.eecs.berkeley.edu/Classes/icdesign/ee240_sp10/lectures/Lecture03_MOS_Models_2up.pdf

    I'm aware of the fact that gm/Id dominates over the square law and the weak inversion model I described, but they do give design insights, and are still extensively used in papers in reference generators.

    Thank you,

    Menghan

Reply
  • MenghanSun

    Hi Andrew,

    Thanks for the reply. I uses spectre mmsim14, virtuoso 616

    This is not a particular model. This is a general fact. For a reasonable channel length (Lmin => 500nm), in saturation, strong inversion, the mosfet exhibits square law characteristics. In subthreshold(weak inversion), the mosfet exhibits exponential characteristics like a BJT, with the addition of "nonideality factor" or "slope factor".

    If we only look at the the exponential term:

    BJT: exp{ [Vgs-VT] / [kT/q]  }

    MOSFET: exp{ [Vgs-VT] / [n*(kT/q)]  }

    This additional n term is typically around 1~1.5

    In the slide 15 in the below link, n is defined as:

    n = 1 + Cdep/Cox

    Which seems like something that would vary with operating point. Maybe I could try to Cdep and Cox in the list of operating point parameter??

     Elad Alon's notes below:

    http://bwrcs.eecs.berkeley.edu/Classes/icdesign/ee240_sp10/lectures/Lecture03_MOS_Models_2up.pdf

    I'm aware of the fact that gm/Id dominates over the square law and the weak inversion model I described, but they do give design insights, and are still extensively used in papers in reference generators.

    Thank you,

    Menghan

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