Abstract

We report on recent progress in AlGaN/GaN HEMT devices grown by plasma-assisted Molecular Beam Epitaxy. While most reports of high power AlGaN/GaN HEMTs have used MOCVD-grown material, recent advances in the growth of AlGaN/GaN HEMT structures by plasma-assisted MBE has enabled devices with performance rivaling the MOCVD-grown HEMTs. In our laboratory, we fabricated submicron HEMTs withf and f_max of 52 and 109 GHz, respectively, from layers grown by plasma-assisted MBE on semi-insulating 6H-SiC substrates. These cutoff frequencies were obtained with gate lengths of 200 nm. The HEMTs were optimized for maximum power output as measured on-wafer using an ATN load-pull setup. At 7 GHz, we recorded a saturated output power density of 6.1 W/mm with 40% PAE and 4.6 dB gain compression. In saturation, a gain of 10.8 dB was still available. The device with a gate periphery of 200 pm was biased at V_DS = 25 V and V_GS = -5 V. At 2 GHz with 1 pm gate length, we recorded a saturated output power density of 8.2 W/mm, with 41% efficiency and 11.2 dB gain. These high power densities are useful for power amplifiers targeted at frequency bands spanning from S- to Ka-band, taking advantage of the high output impedance given by the high breakdown voltage, and the high efficiency of AlGaN/GaN HEMTs. MBE growth has distinct advantages in control of background impurities, uniformity, and interface sharpness. Our devices are unpassivated; we observe minimal drain dispersion or RF drain current compression even without passivation on HEMTs fabricated from optimized layers. We expect to raise ft towards 100 GHz with scaled T-gate length and barrier thickness, enabling high-power amplifiers (few watts) operating up to 50 GHz.

© 2003 Optical Society of America