Patrick Whiting

Field-induced defect morphology in Ni-gate AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors were electrically stressed using off-state high reverse gate biases. In devices demonstrating the largest, most rapid decrease in normalized maximum drain current, defects were found at the gate/AlGaN epilayer interface and characterized using high-angle annular dark-field scanning transmission electron microscopy. These defects appear to be a reaction between the Ni layer of the Ni/Au gate metal stack and the AlGaN epilayer. Additionally, simulations of the electric field lines from the defective devices match the defect morphology. These results provide important insight toward understanding failure mechanisms and improving reliability of Ni-gate AlGaN/GaN high electron mobility transistors.

Under-gate defect formation in Ni-gate AlGaN/GaN high electron mobility transistors

Abstract High electron mobility transistors based on Aluminum Gallium Nitride/Gallium Nitride heterostructures are poised to become the technology of choice for a wide variety of high frequency and high power applications. Their reliability in the field, particularly the reliability of the gate electrode under high reverse bias, remains an ongoing concern, however. Rapid increases in gate leakage current have been observed in devices which have undergone off-state stressing. Scanning Electron Microscopy, scanning probe microscopy, and Transmission Electron Microscopy have been used to evaluate physical changes to the structure of Ni-gated devices as the gate leakage current begins its initial increase. This evaluation indicates the formation of an interfacial defect similar to erosion under the gate observed by other authors. Defect formation appears to be dependent upon electrical field as well as temperature. Transmission Electron Microscopy has been used to demonstrate that a chemical change to the interfacial oxynitride layer present between the semiconductor and gate metal appears to occur during the formation of this defect. The interfacial layer under the gate contact transitions from a mixed oxynitride comprised of gallium and aluminum to an aluminum oxide.

Degradation of sub-micron gate AlGaN/GaN HEMTs due to reverse gate bias

GaN High Electron mobility transistors (HEMTs) were electrically step-stressed under high reverse gate bias conditions. Once a threshold voltage is reached, gate current increases about two orders of magnitude. Though critical voltage was determined to be linear with increasing gate length, electrical simulations show that the maximum electric field was similar at the critical voltage (∼2 MV.cm−1). Electroluminescence and photoluminescence performed on the degraded samples exhibited a decrease in intensity along the periphery of the gate. Transmission electron microscopy shows a thin native oxide layer present under that gate before stressing, and the first stages of gate metal reacting with the underlying AlGaN after stressing.

Nanocrack formation in AlGaN/GaN high electron mobility transistors utilizing Ti/Al/Ni/Au ohmic contacts

Abstract AlGaN/GaN HEMTs are poised to become the technology of choice in RF and power electronics applications where high operating frequencies and high breakdown voltages are required. The alloyed contacting scheme utilized in the formation of the source and drain contacts of these devices affects the conduction of electrons through the 2DEG from the moment of ohmic contact formation onward to operation in the field. Analysis of the ohmic contacts of as-fabricated and electrically stressed AlGaN/GaN HEMTs, via chemical deprocessing and Scanning Electron Microscopy, indicates the presence of cracks oriented along the [11-20] directions, which nucleate at metal inclusions present under the alloyed ohmic source/drain contact metal. Cracks which form at the edges of these contact regions can extend into the channel region. It appears that electrical biasing induces additional growth in the longest cracks present within the channel regions of these devices.