| This thesis investigates the use of N-face GaN-based heterostructures as a promising approach to overcome performance limitations commonly encountered in Ga-face AlGaN/GaN high electron mobility transistors (HEMTs) as their frequency of operation extends into the millimeter-wave and beyond. N-face (0001¯) GaN, with its reversed direction of polarization compared to that of the Ga-face (0001), are well-suited for designing new device structures that address the problems of poor electron confinement and high ohmic contact resistance in highly-scaled GaN transistors. Plasma-assisted molecular beam epitaxy (PAMBE) was the crystal growth technique employed in this research. First, aspects of materials science relevant to the realization of high quality N-face HEMTs grown heteroepitaxially on C-face SiC substrates, such as the nucleation scheme and its impact on the mosaic structure of GaN buffers, were studied and optimized to achieve structural and electrical qualities comparable to those of PAMBE-grown Ga-face GaN. The fabrication and characterization of three N-face metal-insulator-semiconductor HEMT (MIS-HEMT) structures were then presented, where electrostatic analyses and polarization engineering techniques were adopted to delineate the design philosophies. Using non-alloyed ohmic contacts fabricated on HEMT test structures with a highly n-doped cap layer, the principles and epitaxial design strategies for achieving low ohmic contact resistances to the two-dimensional electron gas (2DEG) channels in N-face and Ga-face heterostructures were compared. The benefit offered by N-face GaN was highlighted with a low non-alloyed contact resistance of 0.16 O mm. The use of composition-graded InGaN contact layers was proposed to further reduce contact resistances. Finally, polarity inversion of N-face GaN by PAMBE using an MgxNy interfacial layer was studied, and its potential applications toward transistor designs were discussed. Contrary to the belief of many researchers who have suggested the poor material quality and electrical properties of N-face GaN, this thesis demonstrates the capability to synthesize high-quality N-face heterostructures with room temperature 2DEG mobilities up to 1700 cm2/Vs. It culminates in N-face microwave GaN MIS-HEMTs that demonstrate good rf performance in the C-band (4 GHz) with a continuous-wave output power density and power-added efficiency exceeding 8 W/mm and 70%, respectively, and sets a stage for the future development of N-face GaN millimeter-wave transistors. |
