Model Description

The Agilent Root FET model is a data-based model, rather than a physical or empirical model. It generates a device-specific large-signal model directly from measured DC and small-signal S-parameter data, that accurately represents the terminal characteristics of the device. The model is constructed without circuit simulation or optimization. Use of S-parameter as well as DC data results in a model that can accurately predict nonlinear device performance with frequency dispersion, as a model simulated with a single set of nonlinear current functions cannot. The Agilent Root FET model predicts the device performance as a function of bias, power level, and frequency. It allows circuits containing devices for which measured data exists—but for which there are no good physical or empirical models—to be simulated.

The Agilent Root FET model works for MESFETs and HEMTs (MODFETs). It is scalable to allow simulation of devices with different geometries based on the characterization of a single device. The present implementation is used for positive drain-source bias (Vds 0), which corresponds to the typical operating region.

Based on gate-current and power-dissipation device compliances you provide in software, the data acquisition system calculates the safe operating range for the device. Within this region, the model takes data adaptively at multiple bias points, depending on the specific nonlinearities of the device under test. Measurements are densely spaced in the most nonlinear regions, such as at the knees of the IV curves and the onset of breakdown, and less densely spaced in the linear saturation region.

The model data set of internodal nonlinear current and charge components at each bias point is then mathematically generated and stored. The model is thus generated without the usual need for simulation and optimization as required by all previous empirical models. The Root FET model may thus capture nonlinear device behavior for any MESFET device for which measured data can be taken but for which there are no good physical or empirical models.

The data acquisition system sets the DC bias levels and controls the system hardware to take the DC and S-parameter measurements, based on your inputs. The model generator processes the measured data and generates the lookup tables required by a circuit simulator. It produces data files that are directly readable by the Agilent RF and Microwave Design System (MDS). Both IC-CAP and MDS can be run simultaneously in the X Windows environment. During a simulation, the tabular state-function data from the generated Agilent Root FET model is interpolated using multidimensional spline functions to emulate the terminal characteristics of the device.

The model includes a link to the MNS microwave nonlinear simulator, to allow verification of the model in IC-CAP.