Advanced polarization engineering for gallium nitride-based transistors

This thesis describes the use of polarization engineering to create high-performance and novel Gallium Nitride-based electronic devices.; In recent years, the nitride system of semiconductors has been the focus of a large and fast-increasing global research effort. The unique electrical, optical, and physical properties of this semiconductor family make it ideal for a broad range of applications. The first part of this thesis discusses the development of MBE-grown Ga-polar AlGaN/GaN high electron mobility transistors (HEMTs). Improvements carried out in this work on the epitaxial layer design and fabrication in successive generations of devices have led to record microwave power, efficiency, and linearity. Next, polarization grading as a novel method to obtain three-dimensional carrier channels without bulk impurity doping was explored. Electron mobility in these channels is analyzed theoretically as well as experimentally. Device characteristics of polarization graded channel transistors, PolFETs, are shown to be comparable to AlGaN/GaN HEMTs. Transconductance modification using different channel profiles is shown to be an important advantage of polarization-graded channel transistors.; Most prior work on GaN electronics has been based on Ga-polarity structures. Development of GaN devices with the opposite polarity is discussed in this thesis. Plasma-assisted MBE growth was used to grow high-quality N-polar structures, leading to two-dimensional electron gases with high mobility and charge. Field-effect transistors were fabricated using N-polar heterostructures, and the device characteristics of these are discussed. Bulk trap dispersion and gate leakage were found to be the main problems in these devices. Transistor designs that eliminate these problems are demonstrated in this work, and N-polar transistors with low dispersion and gate leakage were obtained. Growth if thin InN layers on GaN using migration-enhanced epitaxy was also studied. These represent the first detailed study of field-effect transistors on N-polar GaN.