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.
GaN-based high-electron mobility transistors are widely recognized for their exceptional performance at RF and microwave frequencies, and are increasingly being explored for millimeter-wave amplifier applications. An additional application that is critical for future systems is signal switching and routing at millimeter-wave frequencies; this is essential for enabling millimeter-wave wireless communication systems (e.g. 6G and beyond) that require frequency agility and reconfigurability. For this type of RF and mm-wave switch applications, the high carrier concentration and high 2DEG mobility of III-N HEMTs leads to low on resistance and low insertion loss. However, the isolation is limited by off-state capacitance, and nonlinearity of the HEMT limits the power handling capabilities. We have observed that the inclusion of a ferroelectric gate dielectric (using ALD-deposited Hf0.5Zr0.5O2) in the device can significantly enhance performance. By combining polarization engineering of the III-N HEMT with the hysteretic and dispersive polarization characteristics of the ferroelectric gate stack, substantial improvements in the switch figure of merit (FOM=1/2π(RonCoff)) can be achieved. The reduced effective off-state capacitance enabled by ferroelectrics integrated with GaN-based transistors has led to switches with FOM of 2.5 THz. Combining this with advanced processing (e.g. regrowth of source and drain ohmic contacts, gate length scaling), further improvements in performance are expected.
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