Developed on high-quality AlGaN/GaN lateral heterostructures, GaN based high electron mobility transistors (HEMTs) have been regarded as promising candidates for next-generation power switches in numerous power electronics applications owning to their capabilities to deliver low ON-resistance (R_ON), fast switching speed, and high-temperature operation. Over the past few years, significant efforts have been devoted to commercializing GaN power devices, as witnessed by the emergence of several first-generation products released by industrial start-ups as well as leading power semiconductor manufactures. Nevertheless, it still requires intensive efforts to resolve the remaining challenges for widespread acceptance of GaN power devices. To help designers to get the most out of GaN...[ Read more ]

Developed on high-quality AlGaN/GaN lateral heterostructures, GaN based high electron mobility transistors (HEMTs) have been regarded as promising candidates for next-generation power switches in numerous power electronics applications owning to their capabilities to deliver low ON-resistance (R_ON), fast switching speed, and high-temperature operation. Over the past few years, significant efforts have been devoted to commercializing GaN power devices, as witnessed by the emergence of several first-generation products released by industrial start-ups as well as leading power semiconductor manufactures. Nevertheless, it still requires intensive efforts to resolve the remaining challenges for widespread acceptance of GaN power devices. To help designers to get the most out of GaN devices and maximize the system-level benefits, this thesis focuses on application-relevant characterization, implementation, and integration of normally-off GaN power transistors for their adoption in high-performance power converters.

The recently developed AlN passivation technique has shown effective suppression of surface-states-induced current collapse, one of the major challenges facing GaN power devices. However, according to reports of conventional SiN_x-passivated devices, additional trapping of channel hot electrons can significantly deteriorate device dynamic performance. To investigate the effectiveness of AlN passivation in suppressing hot-electron-induced current collapse, a hard switching measurement setup in which hot electrons are generated during ON/OFF switching transients is employed. The dynamic R_ON of the tested E-mode device under hard switching operations shows no further degradation compared to soft switching (i.e., with negligible hot electrons) measurement results, suggesting that hot-electron-induced surface trapping is also suppressed by AlN passivation. Moreover, the small dynamic R_ON degradation with very weak temperature dependence further proves the robustness of AlN passivation in suppressing current collapse, which is attributed to the compensation of trapped electrons by the high-density positive polarization charges in the monocrystal-like AlN passivation layer.

In addition to device-level characterization and analysis for device performance improvement, their interactions with circuit/system need to be investigated to fully leverage the performance advantages of GaN power devices, given their distinct characteristics compared to Si counterparts. With the goal of probing the optimum driving conditions for GaN switches and providing guidelines for their implementation in power converters, this thesis systematically evaluates a 650 V p-GaN gate HEMT. Critical parameters such as R_ON and threshold voltage (V_TH) are evaluated under both static and dynamic (i.e., switching) operating conditions by considering device stability issues including current collapse and V_TH instability. The dynamic R_ON degradation is found to be strongly dependent on the applied ON-state gate voltage V_GS, as a result of a positive shift in V_TH under switching operations. Apart from the characterization of discrete devices, a custom-designed double-pulse test circuit with 400-V, 10-A test capability is built to evaluate their transient switching performance. Optimal gate drive conditions are proposed to: (1) provide sufficient gate over-drive to minimize the V_TH-shift-induced dynamic R_ON degradation, and (2) leave enough headroom to save the device from excessive gate stresses. Gate drive circuit design and board layout considerations are also discussed by taking into account the fast switching characteristics of GaN devices.

The lateral AlGaN/GaN heterojunctions grown on low-cost and highly-scalable Si substrates provide a technology platform that is particularly suitable for high-density integration. To further exploit the advantages of GaN power device technology, the GaN smart power technology platform which allows monolithic integration of low-voltage peripheral circuits with high-voltage GaN switches was proposed several years ago. In this thesis, a GaN pulse width modulation (PWM) integrated circuit (IC) is demonstrated for the first time. Composed of a sawtooth generator and a PWM comparator, the circuit is able to generate 1 MHz PWM signal with its duty cycle modulated over a wide range. Compared to hybrid solutions using discrete GaN power switches and stand-alone Si gate control/drive ICs, this all-GaN solution can deliver improved system performance with suppressed parasitics, reduced board space, and high-temperature operation capability.