| High electron mobility transistors (HEMTs) consisting of aluminum gallium nitride (AlGaN) - gallium nitride (GaN) heterostructures have been characterized by advanced electron microscopy methods including off-axis electron holography, nanoscale chemical analysis, and in situ electrical biasing. The research was divided into three parts: (1) Site-specific sample preparation using focused-ion-beam (FIB) and other complementary techniques; (2) Off-axis electron holography to image electrostatic fields in unbiased and biased devices; and (3) Chemical and micro-structural analysis of as- grown and stressed devices.;The FIB shortcut-liftout method was used to prepare specimens suitable for quantitative electron holography observation. The nature and extent of ion-beam damage was quantified for several ion-beam energies. The impact of ion implantation and specimen amorphization on potential profiles of AlGaN/GaN heterostructures was also evaluated, and methods suitable for reducing and removing FIB damage were explored.;The inelastic mean-free-path (IMFP), an important parameter for local thickness measurements by electron holography, was determined for 200 kiloelectron-volt electrons in GaN. Potential profiles were calculated from reconstructed phase images for HEMT structures prepared by wedge-polishing, argon-milling and FIB using an IMFP of 69.5 nanometers. The profiles matched closely after accounting for the thickness of amorphous layers in the potential calculation.;The potential profiles across AlGaN/GaN heterostructures were mapped in real devices, and characteristic profiles were identified beneath the Schottky gate, insulating SiN layer, and ohmic source and drain. In all cases, the expected built-in piezoelectric field was absent in the AlGaN layer. These devices were also mounted for in situ electrical biasing utilizing a moveable electrode to ground the gate contact. Current-voltage (I-V) curves measured in situ resembled that of a diode, rather than a functioning transistor, suggesting that parasitic channels for electrical current had became active for bias voltages above 3V. Moreover, the HEMT devices failed at biases above 10V due to migration of the FIB-deposited Pt protective layers.;Finally, the site-selectivity of the FIB was utilized to prepare specimens at the location of hot spots identified during reverse-bias step-stress testing. Although a V-type defect was observed at the site of one hot spot, microstructural breakdown was not normally evident. |
