Linearity Characterization and Optimization of Millimeter-Wave GaN HEMTs

This paper presents the first comprehensive study of the linearity characteristics of GaN/SiC high-electron mobility transistors (HEMTs) at millimeter-wave (mmW). Similar size devices from three sources are compared using AM-AM, AM-PM, two-tone, and 16-quadrature-amplitude modulation (16QAM) modulated waveforms measurements at 31.5 GHz. Additionally, error vector magnitude (EVM) and spectrum regrowth [adjacent channel power ratio (ACPR)] data are also presented. The results were measured, using a unique digital waveform system integrated with a mmW (50-GHz) load-pull system, for HEMTs at different classes of operation: A, AB, and B. In all devices, it is observed that a “balanced AB” (b-AB) class of device bias condition where linearity and efficiency are simultaneously optimized. A linearity figure of merit FOM_linearity, based on single-tone power measurements, is defined and calculated for each class. The FOM_linearity closely correlates with linearity performance of the HEMT under two-tone and digitally modulated drive conditions. Compared with class A, at a fixed output power, a b-AB class showed a 5-10-dB improvement in intermodulation distortions, combined with 1.2 × to 1.9 × enhancement in drain efficiency. The characterization methodology and derived FOM_linearity should benefit monolithic microwave integrated circuit (MMIC) designers to optimize the operation of GaN HEMTs. The results also indicate that GaN HEMTs can provide linearity characteristics suitable for applications such as satellite communications, while maintaining high efficiency and power density.