Electrostatic engineering in wide-bandgap semiconductors for high power applications

Abstract

Compared with silicon, wide-bandgap semiconductors offer much higher power efficiency for high-power applications, primarily due to the much higher breakdown field. While the performance advantage has already been offered by vertical SiC and lateral GaN-on-Si devices, even higher promises from vertical GaN devices and ultrawide-bandgap semiconductors such as _-Ga2O3 have not been fully delivered. One of the major reasons is the challenge in managing the high electric field in those materials, without established selective-area p-type doping techniques as in GaN, or effective p-type doping alone as in _-Ga2O3. In this dissertation, we tackle this challenge in vertical GaN and Ga2O3 power devices by investigating novel electric-field management techniques and doping-related issues. The first half the work is centered around leakage-current reduction in power Schottky barrier diodes (SBDs) through the reduced surface field (RESURF) effect, which is arguably necessary for kilovolt-class operations. Two novel device structures are designed and implemented, including i) a trench junction-barrier-Schottky diode (JBSD) structure in GaN that possess the desired RESURF effect without needing for selective-area p-doping, and ii) a trench SBD structure in Ga2O3 that achieves significant leakage-current reduction thus a record-high power figure-of-merit of up to 0.95 GW/cm2 among Ga2O3 power devices, but without the need for p-doping. Furthermore, the ideal reverse leakage characteristics in Ga2O3 SBDs is convincingly identified, enabling the calculation of the practical maximum surface electric field in SBDs – an important concept we unambiguously proposed for the first time. The second half of the work is related to vertical power transistors. Using the MBE-regrowth technique, two novel designs of vertical GaN transistors are demonstrated, including GaN trench MOSFETs with regrown channel and GaN PolarMOS – a VDMOS-like transistor with unique polarization-induced (PI) bulk doping. The main challenge in the regrown lateral p-n junctions in these devices is explicitly revealed by interrogating the regrowth interface, where a significant amount of donor-like charges are found. In addition, sidewall activation and incorporations of PI doping in buried p-type layers are realized for voltage-blocking purposes. In Ga2O3, vertical fin power transistors are developed, showing a high breakdown voltage of over 2.6 kV and a normally-off operation without needing for p-doping. Overall, while p-type doping is extremely beneficial for wide-bandgap vertical power devices, it might not be absolutely necessary, provided that proper electrostatic designs and alternative voltage-blocking junctions are effectively implemented.