Characterizing Low-Frequency Memory Effects using RF Real-Time NVNA Measurements

The GaN high electron mobility transistor (HEMT) technology continues its expansion across the wireless communication industry, owing to its exciting capabilities in terms of power density, thermal management, and frequency of operation. However, this technology also presents some new challenges for the design of RF power amplifiers, as GaN HEMTs are significantly affected by current-reducing memory effects associated with trapping and self-heating. These memory effects introduce low-frequency dispersions which modulate the HEMT’s performance under dynamic large-signal RF operation. In this presentation we will review the device physics associated with these memory effects in GaN HEMTs and the advanced measurement techniques which have been developed to better characterize and model these frequency dispersion effects under dynamic large-signal operation. A particular focus will be placed on a new real-time non-linear vector network analyzer (RT-NVNA) testbed operating at MHz frequencies that enables the instantaneous acquisition of the multi-harmonic large-signal response of GaN HEMTs in operating conditions, such as pulsed class-B mode with arbitrary RF loads. The RT-NVNA allows for the near-isothermal acquisition of the RF dynamic loadlines from pulse to pulse at different biasing conditions to account for different trapping states and operating temperatures. After proper harmonic and intermodulation calibration, the RT-NVNA can also be used to perform real-time active loadpull (RT-ALP) to rapidly characterize the evolution of the device IV characteristics from pulse to pulse using a periodic modulated active load. Modeling results quantifying the impact of trapping upon the GaN HEMT characteristics will be presented.