Some considerations in modeling GaAs MESFETs for portable communication systems

Several approaches in modeling GaAs MESFET devices are considered as they apply to portable communication systems, including: 1) high efficiency power amplifiers where the applied signal exhibits constant envelop modulation, and 2) high linearity small signal amplifiers operating at very low quiescent current levels. For these applications, two device effects which influence model accuracy are investigated, including: a) low-frequency dispersion in output conductance, and b) modulation of threshold voltage with drain-source voltage. Attempts at modeling these two device effects are examined by comparing predicted results to measured output power, harmonic content, drain current, power added efficiency and two-tone third order products of a MESFET under load pull conditions. Comparisons of measured and modeled results suggest modeling threshold voltage modulation with drain-source voltage can lead to more accurate predictions. However, the conventional approach selected for modeling low frequency output conductance dispersion resulted in inaccurate predictions.