Empirical Equivalent-Circuit Modeling of mm-Wave GaN MMIC Varactors by Network Synthesis

Varactors play an important role in realizing tunable networks for reconfigurable microwave monolithic integrated circuits (MMICs), requiring accurate broadband models for their simulation. In this article, a novel method is applied to automatically extract an empirical equivalent-circuit model of varactors starting from measured multibias S-parameter data. A custom network realization approach based on the vector fitting (VF) algorithm and the Bott-Duffin (BD) synthesis is used to automatically construct a network of resistor-inductor-capacitor (RLC) lumped components capable of accurately reproducing the varactor response at high frequency. The method is validated on two gallium nitride (GaN)-integrated varactor technologies for Ka-band applications. The scaling properties of the RLC components obtained by the method are tested with respect to finger width and number of fingers. The implementation of the equivalent circuit in a CAD environment as a network of nonlinear elements is discussed.