1997 Asia Pacific Microwave Conference zyxwvuts 4PO 1-6 z Nonlinear-Dispersive GaAs FET Drain-Current Model for Harmonic Balance Simulation. KW Eccleston Department of Electrical Engineering National University of Singapore SINGAPORE 119260 ABSTRACT: Popular GaAs FET large-signal drain conduction current models depend only on instantaneous terminal voltages, and ignore important phenomena that result in low frequency dispersion. To be valid at microwave frequencies, both the dc and time-varying components of current must be accurately modelled. This paper proposes a GaAs zyxwvut FET drain current model, which includes rate-dependent body and thermal effects, and therefore has the capability to accurately predict both the dc and time-varying components of drain current. Further, this model is particularly suited to harmonic balance simulation of microwave circuits. 1. INTRODUCTION The accuracy of the GaAs FET large-signal drain conduction current model has a significant effect on the accuracy of large-signal simulations of microwave circuits. It is well known that the drain conduction current is both voltage and temperature dependent. Temperature dependence is important as power dissipated by the FET, along with finite thermal resistance, results in significant channel heating. Many of the physical processes that govern the drain conduction current (eg. body effect and surface states) have time-constants of order milliseconds and can only track slowly varying voltages. Likewise, due to significant thermal mass, the thermal time-constant is of the order of milliseconds and hence the channel temperature can only track a slowly varying power dissipation. These rate-dependent phenomena manifest themselves as the so called low frequency drain current dispersion. In the context of operation at microwave frequencies: the channel temperature can only respond to the dc power dissipation; whilst the body effect consists of a component that can only respond to dc voltages, and another to rf voltages. In zyxwvutsrq summary, for microwave circuits, the FET drain current may be written as a function of the instantaneous FET tenminal voltages, their time- averages, and the time-average FET power dissipation. Hence the waveforms of the FET terminal voltages and the instantaneous power dissipation must be considered. Popular drain current models described in the literature [1][2,][3] are dependent only on instantaneous terminal voltages, and as they are derived from dc measurements, they fail to account for the low frequency dispersion, thereby resulting in modelling inaccuracy at RF and microwave frequencies. To resolve this problem, it has been proposed that the drain current nonlinearity parameters should be either extracted from pulsed IN measurements [4] or large-signal RF measurements zyx [5]. However, models derived in this manner will fail to accurately predict the dc drain current, hence dc power dissipation. Recent microwave FET models [6]-[8], which have concentrated on accurate prediction of both the rf and Idc components of drain current, do not explicitly account for rate- dependent effects. Rather, they essentially resolve the drain current into two components: a dc model based on dc measurements, and an RF model derived from the bias behaviour of small- signal g, and go obtained from microwave s- parameters at numerous bias points. The models 717 Authorized licensed use limited to: University of Canterbury. Downloaded on November 29, 2009 at 22:20 from IEEE Xplore. Restrictions apply.