Browsing by Author "Tadjer, Marko"

Now showing 1 - 6 of 6
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    1 kV Self-Aligned Vertical GaN Superjunction Diode
    Ma, Yunwei; Porter, Matthew; Qin, Yuan; Spencer, Joseph; Du, Zhonghao; Xiao, Ming; Wang, Yifan; Kravchenko, Ivan; Briggs, Dayrl P.; Hensley, Dale K.; Udrea, Florin; Tadjer, Marko; Wang, Han; Zhang, Yuhao (IEEE, 2024-01)
    This work demonstrates vertical GaN superjunction (SJ) diodes fabricated via a novel self-aligned process. The SJ comprises n-GaN pillars wrapped by the charge-balanced p-type nickel oxide (NiO). After the NiO sputtering around GaN pillars, the self-aligned process exposes the top pillar surfaces without the need for additional lithography or a patterned NiO etching which is usually difficult. The GaN SJ diode shows a breakdown voltage (B V) of 1100 V, a specific on-resistance ( RON) of 0.4 mΩ⋅ cm2, and a SJ drift-region resistance ( Rdr) of 0.13 mΩ⋅ cm2. The device also exhibits good thermal stability with B V retained over 1 kV and RON dropped to 0.3 mΩ⋅ cm2 at 125oC . The trade-off between B V and Rdr is superior to the 1D GaN limit. These results show the promise of vertical GaN SJ power devices. The self-aligned process is applicable for fabricating the heterogeneous SJ based on various wide- and ultra-wide bandgap semiconductors.
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    10-kV Ga2O3 Charge-Balance Schottky Rectifier Operational at 200 ◦C
    Qin, Yuan; Xiao, Ming; Porter, Matthew; Ma, Yunwei; Spencer, Joseph; Du, Zhonghao; Jacobs, Alan G.; Sasaki, Kohei; Wang, Han; Tadjer, Marko; Zhang, Yuhao (IEEE, 2023-08)
    This work demonstrates a lateral Ga2O3 Schottky barrier diode (SBD) with a breakdown voltage (BV) over 10 kV, the highest BV reported in Ga2O3 devices to date. The 10 kV SBD shows good thermal stability up to 200◦C, which is among the highest operational temperatures reported in multi-kilovolt Ga2O3 devices. The key device design for achieving such high BV is a reduced surface field (RESURF) structure based on the p-type nickel oxide (NiO), which balances the depletion charges in the n-Ga2O3 channel at high voltage. At BV, the chargebalanced Ga2O3 SBD shows an average lateral electric field (E-field) over 4.7 MV/cm at 25 ◦C and over 3.5 MV/cm at 200◦C, both of which exceed the critical E-field of GaN and SiC. The 10 kV SBD shows a specific on-resistance of 0.27 ·cm2 and a turn-on voltage of 1 V; at 200◦C, the former doubles and the latter reduces to 0.7 V. These results suggest the good potential of Ga2O3 devices for mediumand high-voltage, high-temperature power applications.
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    2 kV, 0.7 mΩ·cm2 Vertical Ga2O3 Superjunction Schottky Rectifier with Dynamic Robustness
    Qin, Yuan; Porter, Matthew; Xiao, Ming; Du, Zhonghao; Zhang, Hongming; Ma, Yunwei; Spencer, Joseph; Wang, Boyan; Song, Qihao; Sasaki, Kohei; Lin, Chia-Hung; Kravchenko, Ivan; Briggs, Dayrl P.; Hensley, Dale K.; Tadjer, Marko; Wang, Han; Zhang, Yuhao (IEEE, 2023)
    We report the first experimental demonstration of a vertical superjunction device in ultra-wide bandgap (UWBG) Ga2O3. The device features 1.8 μm wide, 2×1017 cm-3 doped n-Ga2O3 pillars wrapped by the charge-balanced p-type nickel oxide (NiO). The sidewall NiO is sputtered through a novel self-align process. Benefitted from the high doping in Ga2O3, the superjunction Schottky barrier diode (SJ-SBD) achieves a ultra-low specific on-resistance (RON,SP) of 0.7 mΩ·cm2 with a low turn-on voltage of 1 V and high breakdown voltage (BV) of 2000 V. The RON,SP~BV trade-off is among the best in all WBG and UWBG power SBDs. The device also shows good thermal stability with BV > 1.8 kV at 175 oC. In the unclamped inductive switching tests, the device shows a dynamic BV of 2.2 kV and no degradation under 1.7 kV repetitive switching, verifying the fast acceptor depletion in NiO under dynamic switching. Such high-temperature and switching robustness are reported for the first time in a heterogeneous superjunction. These results show the great potential of UWBG superjunction power devices.
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    Design, Fabrication, Characterization, and Packaging of Gallium Oxide Power Diodes
    Wang, Boyan (Virginia Tech, 2024-02-22)
    Gallium Oxide (Ga2O3) is an ultra-wide bandgap semiconductor with a bandgap of 4.5–4.9 eV, which is larger than that of Silicon (Si), Silicon Carbide (SiC), and Gallium Nitride (GaN). A benefit of this ultra-wide bandgap is the high-temperature stability due to the low intrinsic carrier concentration. Another benefit is the high critical electric field (Ec), which is estimated to be from 6 MV/cm to 8 MV/cm in Ga2O3. This allows for a superior Baliga's figure of merit (BFOM) of unipolar Ga2O3 power devices, i.e., they potentially can achieve a smaller specific on-resistance (RON,SP) as compared to the Si, SiC, and GaN devices with the same breakdown voltage (BV). The above prospects make Ga2O3 devices the promising candidates for next-generation power electronics. This dissertation explores the design, fabrication, characterization, and packaging of vertical β-Ga2O3 Schottky barrier diodes (SBDs) and P-N diodes. The power SBDs allow for a small forward voltage and a fast switching speed; thus, it is ubiquitously utilized in power electronics systems. Meanwhile, the Ga2O3 power P-N diodes have the benefit of smaller leakage current, and the diode structure could be a building block for many advanced diodes and transistors. Hence, the study of Ga2O3 Schottky and P-N diodes is expected to provide the foundation for developing a series of Ga2O3 power devices. Firstly, vertical Ga2O3 Schottky and P-N diodes with a novel edge termination (ET), the multi-layer Nickel Oxide (NiO) junction termination extension (JTE), are fabricated on Ga2O3 substrates. This multi-JTE NiO structure decreases the peak electric field (Epeak) at the triple point of device edge when the Ga2O3 diodes are reversely biased. For SBDs, BV reach 2.5 kV, the 1-D junction field reaches 3.08 MV/cm, and the BFOM exceeds 1 GW/cm2. For P-N diodes, BV reaches 3.3 kV, the junction field reaches 4.2 MV/cm, and the BFOM reaches 2.6 GW/cm2. These results are among the highest in Ga2O3 power devices and are comparable to the state-of-the-art vertical GaN Schottky and P-N diodes. Notably, all these diodes are small-area devices. Secondly, large-area (3 mm×3 mm anode size) Ga2O3 Schottky and P-N diodes with high current capability are fabricated to explore the packaging, thermal management, and switching characteristics of Ga2O3 diodes. The same ET is applied for the large-area P-N diode. The fabricated large-area P-N diodes have a turn-on voltage of 2 V, a differential on-resistance (Ron) of 0.2 Ω, and they can reach at least 15 A when measured in the pulse mode. The BV of large-area Ga2O3 P-N diodes varies due to the fabrication non-uniformity, but the best device achieves a BV of 1.6 kV, standing among the highest values reported for large-area Ga2O3 diodes. Also, the large-area Ga2O3 SBDs with similar current rating but with a FP ET are fabricated mainly for the packaging and thermal management studies. Thirdly, medium-area Ga2O3 P-N diodes with a current over 1 A and a higher yield of BV are fabricated to evaluate the JTE's capacitance and switching characteristics. The JTE accounts for only ~11% of the junction capacitance of this 1 A diode, and the percentage is expected to be even smaller for higher-current diodes. The turn-on/off speed and reverse recovery time of the diode are comparable to commercial SiC Schottky barrier diodes under the on-wafer switching test. These results show the viability of NiO JTE for enabling a fast switching speed in high-voltage Ga2O3 power devices. Fourthly, the fabricated large-area Ga2O3 diodes are packaged using silver sintering as the die attach. The sintered silver joint has higher thermal conductivity (kT) and better reliability as compared to the solder joint. Due to the low kT of Ga2O3 material, junction-side-cooled (JSC) packaging configuration is necessary for Ga2O3 devices. For the packaged device, its junction-to-case thermal resistance (RθJC) is measured in the bottom-side-cooled (BSC) and junction-side-cooled (JSC) configuration by the transient dual interface method according to the JEDEC 51-14 standard. The RθJC of the junction- and bottom-cooled Ga2O3 SBD is measured to be 0.5 K/W and 1.43 K/W, respectively. The former RθJC is lower than that of similarly-rated commercial SiC SBDs. This manifests the significance of JSC packaging for the thermal management of Ga2O3 devices. Fifthly, to evaluate the electrothermal robustness of the packaged Ga2O3 devices, the surge current capability of JSC packaged Ga2O3 SBDs are measured. The Ga2O3 SBDs with proper packaging show high surge current capabilities. The double-side-cooled (DSC) large-area Ga2O3 SBDs can sustain a peak surge current over 60 A, with a ratio between the peak surge current and the rated current superior to that of similarly-rated commercial SiC SBDs. These results show the excellent ruggedness of Ga2O3 power devices. Finally, a Ga2O3 integrated diode module consisting of four single-diode sub-modules is designed and fabricated. For many power electronics applications, high current is desired; however, for emerging semiconductors, the current upscaling is difficult by directly increasing the device area because of the limitation of heat extraction capability and the limited material/processing yield. Here we explore the paralleling of multiple Ga2O3 P-N diodes to increase the current level. For each sub-module, the JSC packaging structure is used for heat extraction, and a metal post is sintered to the anode for electric field (E-field) management. RθJC is measured to be 1 W/K for each sub-module. On-board double-pulsed test is performed for both the sub-module and the full module. The sub-module and full module demonstrate 400 V, 10 A and 150 V, 70 A switching capabilities, respectively. This is the first demonstration of Ga2O3 power module and shows a promising approach to upscale of the power level of Ga2O3 power electronics. In addition to Ga2O3 device study, a research is conducted to explore the chip size (Achip) minimization for wide-bandgap (WBG) and ultra-wide bandgap (UWBG) power devices. Achip optimization is particularly critical for WBG and UWBG power devices and modules due to the high material cost. This work presents a new, holistic, electrothermal approach to optimize Achip for a given set of target specifications including BV, conduction current (I0), and switching frequency (f). The conduction and switching losses of the device are considered, as well as the heat dissipation in the chip and its package. For a given BV and I0, the optimal Achip, Wdr, and Ndr show strong dependence on f and thermal management. Our approach offers more accurate cost analysis and design guidelines for power modules. In summary, this dissertation covers the design, fabrication, characterization, and packaging of Ga2O3 Schottky and P-N diodes, with the aim to advance Ga2O3 devices to power electronics applications. This dissertation addresses many knowledge gaps on Ga2O3 devices, including the voltage upscaling (ET), current upscaling (large-area device fabrication, packaging, and thermal management), and their concurrence (module demonstration), as well as the circuit-level switching characterizations.
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    Epitaxial Gallium Oxide Heterojunctions for Vertical Power Rectifiers
    Spencer, Joseph Andrew (Virginia Tech, 2024-06-03)
    At the heart of all power electronic systems lies the semiconductor, responsible for passing large amounts of current at negligible power losses in the on-state, while instantaneously switching to withstand high voltages in the off-state. For decades silicon (Si) has dominated nearly all aspects of electronic systems including power. As importunity for efficiency at higher power and fast switching speeds grows, the environments with which these systems are being tasked to operate in has also increased in rigor. This has placed semiconductors at the forefront of innovation as novel materials are being explored in hopes of meeting the demands for the future of power electronics. This exploration of novel materials for power electronics has come to fruition as the performance limits of narrow bandgap (EG) materials such as Si (1.1 eV) have been reached. The EG is a key measure of a materials ability to operate at high voltages and within high temperature environments. This is due to the direct relationship of the EG to the critical field strength which enables increased performance beyond that of narrow band gap materials such as Si and gallium arsenide. Wide bandgap (WBG) materials such as silicon carbide (SiC) and gallium nitride (GaN) with EG 3.3 eV and 3.4 eV, respectively, have emerged within the power electronics field to offer increased breakdown voltages (VBR) at lower on-resistances. However, ultrawide bandgap (UWBG) devices possess greater potential with superior performance limits in comparison to SiC and GaN. Ga2O3 (4.8 eV) is the only UWBG semiconductor with melt-growth capabilities that has already demonstrated research grade wafers up to 6" in diameter. Ga2O3 is also advantaged by the ability to grow thick, lowly-doped homoepitaxial drift regions from methods such as halide vapor phase epitaxy (HVPE) and metal organic chemical vapor deposition (MOCVD). This makes Ga2O3 a prime candidate for vertical power rectifiers as thick, high quality drift regions are a necessity for high voltage devices such as the PN diode, junction barrier Schottky (JBS) diode, merged-PiN-Schottky (MPS) diode, and Schottky barrier diode (SBD). However, Ga2O3 exhibits a lack of p-type conductive that arises from an absence of dispersion within the valence band maximum. This has caused researchers to abandon the idea of homojunction devices that Si, SiC, and GaN devices benefit from; shifting to a heterojunction approach where NiO (3.7 eV) provides the source of p-type conductivity. This complicates fabrication and device characterization particularly for the Ga2O3 JBS diode where an etched Ga2O3-NiO heterojunction has thus far been unreported throughout the literature. This work investigates the numerous individual aspects that comprise an etched Ga2O3 heterojunction device which include the etching method, post etch damage removal and its impact on electrical performance, and ohmic and Schottky contacts critical for a JBS diode; all culminating in the demonstration of a JBS and MPS diodes. We also report our investigations into co-doping of Ga2O3 that yield degenerately doped epitaxial layers with record mobility (μ) values. While not directly correlated with Ga2O3-NiO heterojunction devices, this study lays the ground work for semi-insulating Ga2O3 depletion into unintentionally doped (UID) n-type Ga2O3.
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    Thermal management and packaging of wide and ultra-wide bandgap power devices: a review and perspective
    Qin, Yuan; Albano, Benjamin; Spencer, Joseph; Lundh, James Spencer; Wang, Boyan; Buttay, Cyril; Tadjer, Marko; DiMarino, Christina; Zhang, Yuhao (IOP Publishing, 2023-03)
    Power semiconductor devices are fundamental drivers for advances in power electronics, the technology for electric energy conversion. Power devices based on wide-bandgap (WBG) and ultra-wide bandgap (UWBG) semiconductors allow for a smaller chip size, lower loss and higher frequency compared with their silicon (Si) counterparts, thus enabling a higher system efficiency and smaller form factor. Amongst the challenges for the development and deployment of WBG and UWBG devices is the efficient dissipation of heat, an unavoidable by-product of the higher power density. To mitigate the performance limitations and reliability issues caused by self-heating, thermal management is required at both device and package levels. Packaging in particular is a crucial milestone for the development of any power device technology; WBG and UWBG devices have both reached this milestone recently. This paper provides a timely review of the thermal management of WBG and UWBG power devices with an emphasis on packaged devices. Additionally, emerging UWBG devices hold good promise for high-temperature applications due to their low intrinsic carrier density and increased dopant ionization at elevated temperatures. The fulfillment of this promise in system applications, in conjunction with overcoming the thermal limitations of some UWBG materials, requires new thermal management and packaging technologies. To this end, we provide perspectives on the relevant challenges, potential solutions and research opportunities, highlighting the pressing needs for device-package electrothermal co-design and high-temperature packages that can withstand the high electric fields expected in UWBG devices.