Admittance of an MOS device with interface charge inhomogeneities

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Abstract
Two small‐signal ac equivalent circuits relating the experimental admittance of an entire MOS device to the interface‐state admittance are derived from a small‐signal analysis of the three‐dimensional Poisson equation for the device. One equivalent circuit, the original Nicollian‐Goetzberger equivalent circuit, is shown to be valid for a random interface charge distribution, provided the device is operated in depletion where the ac‐induced change in surface potential fluctuations is small compared to the fluctuations themselves. The other is shown to be valid for a very smooth quasi‐uniform interface charge distribution.