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Is the device capacitance bias-dependent or independent?

Alex Liao
Hi, I would like to know if the device intrinsic capacitance like Cgs bias-dependent or independent. Giving say, bias 400mV or 500mV (common source circuit with Vgs = 400mV or 500mV) for 65nm technology, is the Cgs the same or different and why？

Thanks,
Alex
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  • Dear Alex,

     Yes,  the device intrinsic capacitance Cgs is bias-dependent. Intuitively, the gate source capacitance is a function of how depleted the channel is. As such, more or less charge is exposed as the gate source voltage is varied and hence the capacitance is a function of the DC operating point. 

     

    However, since the specific point in the MOS structure at which the "gate" starts and the interconnect to the "gate" ends, there is also a bias voltage independent capacitance that will be due to routing capacitance to the point where the "gate" starts.

     

    I hope this helps Alex,

     

    Shawn 

Reply

  • Dear Alex,

     Yes,  the device intrinsic capacitance Cgs is bias-dependent. Intuitively, the gate source capacitance is a function of how depleted the channel is. As such, more or less charge is exposed as the gate source voltage is varied and hence the capacitance is a function of the DC operating point. 

     

    However, since the specific point in the MOS structure at which the "gate" starts and the interconnect to the "gate" ends, there is also a bias voltage independent capacitance that will be due to routing capacitance to the point where the "gate" starts.

     

    I hope this helps Alex,

     

    Shawn 

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