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...[
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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.
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