| The rapid emergence of nitride-based semiconductor devices for high-temperature, high-power, and high-frequency electronics applications has been fueled by the unique material properties of this semiconductor system. Essential to their large-scale incorporation into integrated circuits and systems is a detailed understanding of the beneficial and detrimental material properties of these relatively new semiconductor alloys.; This dissertation first focuses on material properties that currently limit AlGaN/GaN heterostructure field-effect transistor (HFET) devices and how some of these issues can be overcome. Leakage current mechanisms in HFET structures grown by metal-organic chemical vapor deposition were investigated using extensive electrical characterization, two-dimension numerical simulations, and analytical modeling. Vertical tunneling was found to be the dominant leakage current mechanism, but additional leakage mechanisms also contribute to the total leakage current. Dislocation-related leakage current paths in AlGaN/GaN heterostructures grown by molecular beam epitaxy, which are the primary source of leakage current in such structures, were studied using conductive atomic force microscopy (AFM), and were blocked by the growth of thin insulating layers over the leakage paths when a voltage was applied between an AFM tip and the sample, greatly reducing the leakage current in Schottky diodes fabricated on AFM-modified surfaces compared to diodes fabricated on unmodified surfaces. A large-scale, electrochemical surface treatment based on similar physical principles was developed, which achieved a similar reduction in leakage current. Spatial variations in the threshold voltage of an AlGaN/GaN heterostructure were studied using scanning capacitance spectroscopy and were related to the surface potential of threading dislocations and compositional and barrier thickness variations. Finally, gate-drain capacitance and conductance measurements of an AlGaN/GaN HFET were used to characterize trap states in the device.; In the second part of this dissertation, the exceptionally large polarization charge present in nitride alloys is discussed. The polarization charge at AlGaN/GaN heterostructure interfaces was directly measured using capacitance-voltage carrier profiling, and the measured values were in reasonable agreement with theoretically predicted values. Using analytical electrostatic dipole energy calculations, clustering of certain types of atoms in nitride alloys was shown to be energetically favorable due to the interactions of the polarization-induced dipole moments at each unit cell. |
