We review advances in GaN-based microwave power field-effect-transistors (FETs). Evolution in device technology included metal-semiconductor-field-effect-transistors (MESFETs), heterostructure-field-effect-transistors (HFETs), modulation-doped-field-effect-transistors (MODFETs) or high-mobility-transistors (HEMT), HEMTs with high Al contents, HEMTs with gate recess and GaN-channel HEMTs grown on SiC substrates. The power density was first reported as 1.1 W/mm at 2 GHz using an AlGaN/GaN HEMT structure grown on sapphire substrate, and was subsequently improved to 1.5-1.7 W/mm at 4-10 GHz by refinement in device structure and processing techniques. This was advanced to 2.6-3.3 W/mm at 8-18 GHz by adopting a high-Al-content AlGaN barrier layer. Success in gate recess helped to further increase the power density of these GaN HEMTs on sapphire substrates to 4.6 W/mm at 6 GHz. Substrate replacement of sapphire by SiC, for excellent thermal dissipation, has boosted performance to 6.9 W/mm at 10 GHz, which is higher than GaAs-based FETs by a factor of 6. Device periphery was scaled up to obtain high total output power. On one hand, GaN HEMTs on sapphire, using a flip-chip bonding technology for thermal management, have generated 7.6 W at 4 GHz. On another hand, GaN HEMTs on SiC, taking advantage of the high substrate thermal conductivity, have achieved 9.1 W at 7.4 GHz. Two types of initial GaN-based power amplifiers were also demonstrated using a flip-chip IC scheme. The transistors used were 0.7 to 0.8-µm-long-gate GaN HEMTs. Bandwidths of 1-8 GHz and 3-9 GHz were achieved with gains up to 11.5 dB. The output power levels ranged from 3.2 to 4.6 W using devices with 2 and 3-mm gate peripheries, which were higher than that achievable with GaAs-based HEMTs of the same size by a factor of 2. Traps in the device structure currently limit performance of most GaN FETs. These traps cause dispersion in the I-V characteristics, which increases knee voltage and reduces channel current under RF gate drive. However, they are believed to be not inherent in the GaN semiconductor system and can be minimized as the technology matures.

In this paper we review our recent work developing the growth of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on SiC (0001) by plasma-assisted molecular beam epitaxy (PA-MBE). State-of-the-art AlGaN/GaN HEMTs have been achieved using MBE-grown material. Buffer leakage was an important limiting factor for early devices. We have shown that by appropriately controlling the Al/N flux ratio during growth of the nucleation layer on SiC(0001), low-leakage GaN buffers can be subsequently grown. In addition, a "modulated growth" technique was developed to achieve large area uniformity and surface morphology control. High-performance HEMTs were fabricated utilizing these two techniques. On 200 nm gate-length devices, at 4 GHz an output power density of 8.4 W/mm was obtained with a power-added efficiency (PAE) of 67% at a drain bias of 30 V. At a higher drain bias (42 V), 13.7 W/mm with a PAE of 55% was achieved.