A compact nonlinear model for coplanar waveguides on silicon substrates

A wideband nonlinear equivalent circuit model for quasi-TEM coplanar waveguide (CPW) transmission lines fabricated on low-resistivity Si substrates is proposed and verified experimentally. The model includes nonlinear bias-dependent junction conductances and capacitances, which enable the model to scale with substrate doping concentration and transmission line geometry. Numerical calculation of the CPW capacitance, based on 2D solutions of Poisson's equation, as well as experimental investigations of the dependence of model parameters on substrate doping type (both n- and p-type) and doping concentration have been performed. Measurements of typical devices show excellent agreement between the model prediction and measured transmission line S-parameters from 100 MHz to 10 GHz. Analysis of the model indicates that a full back-to-back metal-semiconductor junction contact model is required for CPWs on n-type substrates, while the higher Schottky barrier height of typical metal contacts to p-type Si permits a simpler one-sided junction model for CPWs on p-type substrates.