THlE-3 EMPIRICAL LOAD-LINE CAPACITANCE MODELS FOR HEMT Y.C. Leong’ and S. Weinreb2 U. of Massachusetts, 619 Lederle GRC, Amherst, zyxw MA 01003 2Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109-8099 zy 1 ABSTRACT Models for describing the gate-source and gate-drain capacitances’ variation along resistive load line have been proposed. They are charge conservative and consistent with small-signal model at bias points along the load line. Extraction procedure for the models’ parameters is fast and intuitive. The models can be implemented easily in most circuit simulator programs. INTRODUCTION The gate-source and gate-drain capacitances of HEMT have long been determined to have dual voltage dependency. Expressions for these capacitances as hnctions of two voltages have been published. However, small-signal linearization of the charge hnctions that are derived from these capacitance fbnctions will introduce transcapacitances zyxwvut [ 11. These extra elements cause discrepancy between the large- signal and small-signal models. A recent transistor model proposed in [2] is consistent and charge conservative. However, it requires a new topology for the intrinsic small-signal equivalent circuit model. Very often, the constant capacitance or junction capacitance models are still used. But in these two cases, variation of the capacitances with output voltage swing is not modeled. MODEL DESCRIPTION In this paper, empirical models for the gate- source and gate-drain capacitances will be proposed. These models have only single voltage dependency and they track the capacitances’ variation along a resistive load line in the IV plane of the HEMT. In this way, the influence of the output voltage on the capacitances can be modeled by using only the local voltages of the capacitors. They are charge conservative and the nonlinear model will be consistent with the common small-signal model at bias points that are along a resistive load line in the IV plane. The extraction procedure for the models’ parameters is straightforward and can be done using commonly available spreadsheet programs. As the capacitance hnctions are only one dimensional, the time needed to extract the capacitance models is very fast and the fitting is of good accuracy. The models also give zy an intuitive picture of the behavior of the FET during large-signal excitation since fitting is done directly with the measured capacitances’ variations instead of the S-parameters. These models can be implemented easily in most circuit simulators as simple charge sources across the capacitors’ terminals or as single-voltage dependent capacitors. The load-line models had been used to analyze a 47 zyxw GHz HEMT frequency tripler and good agreement has been obtained between the measured and modeled results. 0-7803-5687-X/00/$10.00 zyxwvutsrqp 0 2000 IEEE 1385 2000 IEEE M7T-S Digest