Accuracy of Surface-Potential-Based Long-Wide-Channel MOS Transistor Compact Models
B.B. Jie and C-T Sah
University of Florida, US
surface potential compact model, bulk charge approximations, percentage-RMS-deviation, non-compact baseline model
Frequently asked questions are addressed, “How accurate are the approximate long-and-wide-channel MOS transistors baseline models that have been used to develop the compact models for computer-aided circuit designs?” Three commonly used surface-potential (US=q*PSIS/kT) approximations of the bulk-charge, QB (≡ QP for p-Base of nMOST), are evaluated: QB (i) (US)^(1/2) and (ii) (US−1)^(1/2), and (iii) [US−1+exp(−US)]^(1/2). Several new approximations are also evaluated. Self-consistent remote charge-neutrality boundary-condition and minority carriers are taken into account. Percentage-deviation and percentage-RMS-deviation of the DC drain current and the drain- and trans-conductances from the non-compact baseline model (long-and-wide-channel, and space-constant impurity-concentration and oxide-thickness) are computed for the engineering ranges of the body impurity concentration and oxide thickness (10^16 to 10^19 cm−3 and 1 to 30 nm). Approximation (i) shows significant deviations, ~16% at threshold diverging rapidly with deepening into the subthreshold range. Approximations (ii) and (iii) show a few percent (1% to 2%) deviations in both inversion and subthreshold ranges, while (ii) diverges as US →1+ or PSIS → kT/q above flatband. In the accummulation range, US<0, better approximations are needed.
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Nanotech 2006 Conference Program Abstract