State dependency, low-frequency dispersion, and thermal effects in microwave III-V HEMTs

An AlGaN/GaN HEMT- and an InAlAs/InGaAs mHEMT technology, both with a gate-length of 100 nm, are investigated w.r.t. state dependency vs. average gate and drain voltages, and low-frequency (LF) dispersion, and the separation of thermal effects is demonstrated. Based on a comprehensive DC-CW and pulsed-RF small-signal characterization, it is shown that the GaN HEMT shows all three effects, while the mHEMT is nearly free of state dependency. The description of the LF dispersion using classical large-signal FET models is compared to the recently proposed integral-transform (ITF) model. A product separation approach for the thermal effects and the extraction of thermal parameters via simultaneous equations is described. A new formulation of the ITF model is capable of describing all three above effects in pulsed-RF and even in CW load-pull operation conditions.