| GaN and its related alloys are a very important class of semiconductors for electronic applications. After more than a decade of development, AlGaN/GaN high electron mobility transistors (HEMTs) are showing excellent power amplification performance at frequencies ranging from the S-band to the W-band. The vast majority of electronic devices have been fabricated on the (0001) metal-polar crystal plane because of the high polarization along this axis and the relative ease of crystal growth along this orientation. The N-polar, or (0001¯) orientation has the polarization reversed, which allows us to explore novel device designs which address the problems of high ohmic contact resistance and poor electron confinement which degrade the high-frequency performance of metal-polar HEMTs. However, N-polar HEMTs are relatively unstudied primarily because high-quality materials have been very difficult to grow. This thesis will explore the growth of N-polar GaN and its alloys by metal organic chemical vapor deposition. Atomically smooth, high-quality N-polar GaN was achieved on sapphire and SiC through the use of vicinal substrates. We also demonstrated AlGaN/GaN heterojunctions and 2DEGs with mobility higher than 1500 cm 2V-1s-1, and developed a theoretical model of the electron mobility in N-polar HEMT structures which matched experimental results well. The growth of N-polar InAlN, a promising material to use in the barrier layer of HEMTs, was studied comprehensively in this thesis. Finally, we describe the development of optimized N-polar AlGaN/GaN HEMT structures with 67% power-added efficiency at 4 GHz, and InAlN/GaN HEMTs with 1.4 A/mm of current. |
