MOS Capacitor

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MOS capacitances

  • oxide capacitance between the gate and the channel \(C_1=WLC_{ox}\)
  • depletion capacitance between the channel and the substrate
  • junction capacitance between the source/drain areas and the substrate
    • The value of \(C_{SB}\) and \(C_{DB}\) is a function of the source and drain voltages with respect to the substrate

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The gate-bulk capacitance is usually neglected in the triode and saturation regions because the inversion layer acts as a "shield" between the gate and the bulk.

Temperature Dependence of Junction Diode CV

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where TCJ and TCJSW are positive

https://cmosedu.com/cmos1/BSIM4_manual.pdf

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D=S=B varactor

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Inversion-mode (I-MOS)

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Accumulation-mode (A-MOS)

NMOS in NWELL, aka NMOS in N-Well varactor

Notice: S/D and NWELL are connected togethor in layout

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PDK varactor

nmoscap: NMOS in N-Well varactor

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  • Base Band MOSCAP model (nmoscap) is built without effective series resistance (ESR) and effective series inductance (ESL) calibrations, which is for capacitance simulation only
  • LC-Tank MOSCAP model (moscap_rf) is for frequency-dependent Q factor and capacitance simulations

MOS Device as Capacitor

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Voltage dependence

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  • capacitance of MOS gate varies nonmonotonically with \(V_{GS}\)

  • "accumulation-mode" varactor varies monotonically with \(V_{GS}\)

Inverter capacitance

invCap

reference

R. L. Bunch and S. Raman, "Large-signal analysis of MOS varactors in CMOS -G/sub m/ LC VCOs," in IEEE Journal of Solid-State Circuits, vol. 38, no. 8, pp. 1325-1332, Aug. 2003, doi: 10.1109/JSSC.2003.814416.

T. Soorapanth, C. P. Yue, D. K. Shaeffer, T. I. Lee and S. S. Wong, "Analysis and optimization of accumulation-mode varactor for RF ICs," 1998 Symposium on VLSI Circuits. Digest of Technical Papers (Cat. No.98CH36215), 1998, pp. 32-33, doi: 10.1109/VLSIC.1998.687993. URL: http://www-smirc.stanford.edu/papers/VLSI98s-chet.pdf

R. Jacob Baker, 6.1 MOSFET Capacitance Overview/Review, CMOS Circuit Design, Layout, and Simulation, Fourth Edition

B. Razavi, Design of Analog CMOS Integrated Circuits 2nd