Alan F. Wright

Materials physics and device development for improved efficiency of GaN HEMT high power amplifiers.

GaN-based microwave power amplifiers have been identified as critical components in Sandias next generation micro-Synthetic-Aperture-Radar (SAR) operating at X-band and Ku-band (10-18 GHz). To miniaturize SAR, GaN-based amplifiers are necessary to replace bulky traveling wave tubes. Specifically, for micro-SAR development, highly reliable GaN high electron mobility transistors (HEMTs), which have delivered a factor of 10 times improvement in power performance compared to GaAs, need to be developed. Despite the great promise of GaN HEMTs, problems associated with nitride materials growth currently limit gain, linearity, power-added-efficiency, reproducibility, and reliability. These material quality issues are primarily due to heteroepitaxial growth of GaN on lattice mismatched substrates. Because SiC provides the best lattice match and thermal conductivity, SiC is currently the substrate of choice for GaN-based microwave amplifiers. Obviously for GaN-based HEMTs to fully realize their tremendous promise, several challenges related to GaN heteroepitaxy on SiC must be solved. For this LDRD, we conducted a concerted effort to resolve materials issues through in-depth research on GaN/AlGaN growth on SiC. Repeatable growth processes were developed which enabled basic studies of these device layers as well as full fabrication of microwave amplifiers. Detailed studies of the GaN and AlGaN growth of SiC were conducted and techniques to measure the structural and electrical properties of the layers were developed. Problems that limit device performance were investigated, including electron traps, dislocations, the quality of semi-insulating GaN, the GaN/AlGaN interface roughness, and surface pinning of the AlGaN gate. Surface charge was reduced by developing silicon nitride passivation. Constant feedback between material properties, physical understanding, and device performance enabled rapid progress which eventually led to the successful fabrication of state of the art HEMT transistors and amplifiers.

High Al-Content AlInGaN Devices for Next Generation Electronic and Optoelectronic Applications

Great strides have been made in the development of ultraviolet LED materials and devices. Power levels in the near UV (below 390 nm) have been improved from the 10 W to the 1 mW level through improvements in the growth and design of AlInGaN alloys. High frequency AlGaN/GaN HEMTs have been developed with ft of 65 GHz and fmax of 85 GHz, all while attaining breakdown voltage greater than 100 V. A new breakthrough in the lateral overgrowth of GaN materials promises to further improve these devices.