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Wide (and ultra-wide) bandgap III-N semiconductors are promising for electronic devices operating at high power levels and in harsh environments, as well as for sensors such as UV detectors. The promise of these materials derives from a combination of their excellent carrier transport properties and the ability to operate at high internal electric fields. However, many current-generation devices do not fully exploit the material limits, and thus performance is below the fundamental performance limits expected from the material properties. Recent work on polarization-graded structures for internal electric field mitigation to enhance the breakdown voltage in HEMTs, cost-effective edge termination strategies for vertical power devices, and devices exploiting impact ionization and avalanche in GaN will be discussed. For example, we find that the use of polarization-grading can decrease the peak electric field in the channel, increase the breakdown voltage, and improve the power scaling of III-N based HEMTs, without the use of field plates that limit high-frequency performance; experimentally-validated power-added efficiency of 50% at 94 GHz has been achieved. In vertical devices, device high-field operation is often limited by edge effects; we report a strategy for edge termination that provides a large process window that is tolerant of both fabrication processing and epitaxial layer thickness and doping variations, and enables robust avalanche operation to be achieved in practice. In addition to increased breakdown voltage, the ability to harness impact ionization and avalanche for device functionality is also critical for avalanche photodiodes and negative-resistance oscillators such at IMPATT diodes. We report the recent demonstration of experimentally-measured negative resistance at microwave frequencies from GaN-based IMPATT diodes, illustrating direct exploitation of the high-field operation of GaN pn junctions for advanced functionality. These approaches are amenable to extension to ultra-wide bandgap III-N materials, particularly for applications in quantum sensing and ultra-high power density electronics.
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