High-frequency network properties of MOS transistors including the substrate resistivity effects

The small-signal differential equations describing the intrinsic high-frequency characteristics of MOS transistors are derived under three basic modes of signal application: gate excitation, substrate excitation, and combined gate-substrate excitation. These equations are shown to be analogous to those of a double RC transmission line having a uniformly distributed common resistance but two separate capacitances distributed nonuniformly. High-frequency device admittances are calculated in terms of those of the analog RC transmission line using the method of "piecewise" uniformity for the capacitance distributions. Useful expressions are derived for the various Y-parameters which are explicitly related to the more readily measurable low-frequency input capacitance and transconductance parameters which, in turn, are related to the basic device physical parameters. The admittance expressions clearly indicate the influence of the substrate resistivity both on the forward transfer admittance magnitudes at low and high frequencies and on the input conductance at high frequencies. The intrinsic Y-parameters are combined with the associated extrinsic RC networks and presented in the form of equivalent circuits. Results of UHF admittance measurements on representative n-channel devices are given which support the overall validity of the proposed equivalent circuit models.