Wide Bandgap Semiconductor Power Research Articles

IEEE ITRW Working Group Position Paper-Materials and Devices: WBG and UWBG Materials and Devices Are Examined in a New Working Group

The International Technology Roadmap for Wide-Bandgap Power Semiconductors (ITRW) materials and devices working group considers the materials science of wide-bandgap (WBG) and ultrawide-bandgap (UWBG) semiconductors in addition to device design, fabrication, and evaluation. Its purpose is to formulate a long-term, international roadmap for WBG and UWBG materials and devices consistent with the packaging and applications working groups of ITRW.

Natural‐air‐cooled 5 kVA single‐phase GaN inverter with paralleled multilayer PCB magnetics

In this study, a systematic approach to the design and development of a high-efficiency, natural-air-cooled, single-phase inverter with a multilayer high-current printed-circuit board (PCB) magnetics is presented. The size and efficiency of inverters implemented with the silicon transistor technology have almost reached a certain limit. The use of wide bandgap power semiconductors, such as the silicon carbide metal–oxide–semiconductor field-effect transistor and the gallium nitride (GaN) enhancement-mode (e-mode) transistor not only pushes further the efficiency limits, but also shrinks the inverter size. A new approach is proposed here in order to obtain a high-efficiency inverter which relies basically upon the derivation of analytical expressions for the inverter losses as a function of the inverter modulation index, for the optimum transistor and highest switching frequency pair selection, and the design of the output filter based on a paralleled, multilayer PCB magnetics. In the developed 5 kVA single-phase full-bridge inverter, GaN e-mode transistors are used to minimise the inverter losses and to decrease the cooling requirement, and size of the system. The proposed systematic design approach has been verified on the implemented 5 kVA, 50 kHz, naturally-cooled GaN inverter, with a power density of 2.7 W/cm 3 (44.3 W/inch 3 ), and a full-load efficiency of 98%.

Key Components of Modular Propulsion Systems for Next Generation Electric Vehicles

The objective of this paper is to provide an overview of emerging technologies for modular power converter architectures for electric vehicles. Nowadays, the most common electrical drive-train architecture exhibits one single inverter which is directly tied to the battery. As a consequence, only one high-voltage battery module can be applied and the dc-link voltage of the inverter and its apparent power rating is directly dependent on the available battery voltage. To overcome this restriction, modern power converter architectures with a higher degree of freedom have been proposed. These architectures exhibit modular dc-dc converters to allow different battery technologies to be linked to drive inverters operating independently from each other. To make this development feasible, new components and technologies are evolving which enhance the efficiency over mission cycles while ensuring further integration of the power-converter architectures. Wide-bandgap power semiconductors enable high switching frequencies and miniaturization of passive devices. Smart topology enhancements and control methods allow a significant loss reduction, in particular at light loads, resulting in a higher efficiency of the drive train over the entire driving cycle. Highly integrated bidirectional battery charger systems with intelligent charging strategies inhibit battery degradation and provide opportunities for grid stabilization. It is demonstrated how these technologies are realized and implemented to contribute to the development of future electric vehicles.

The Past, Present, and Future of Power Electronics Integration Technology in Motor Drives

The physical integration of power electronics and electric machines to form integrated motor drives (IMDs) eliminates the need for special enclosures and connecting cables in order to achieve mass, volume, and cost savings. The objective of this paper is to examine the future of integrated motor drive technology by first reviewing the history of IMD products from the 1960s to today, highlighting both the reasons for their success as well the significant technical obstacles that they had to overcome. Special attention is directed to the application of IMD technology to electric vehicle traction motor drives during the past 15 years. A long-term vision for IMDs is presented that calls for embedding the drive electronics directly inside the machine enclosure. In keeping with this vision, wide-bandgap (WBG) power semiconductor switches (SiC and GaN) offer exciting prospects for shrinking the size of power converters and simplifying their cooling requirements. New concepts for applying this WBG technology to IMDs are introduced, including revived interest in PWM current-source inverters. In the concluding section, a variety of other promising technologies are introduced that will be critical to realizing the full potential of integrated motor drives.

Emerging Trends in Silicon Carbide Power Electronics Design

The emergence of wide bandgap power semiconductor devices has opened the possibilities of improved electrical performance and power density. Advanced research into wide bandgap power electronics also includes advances in integrated circuit design, semiconductor device modeling, 3D electronic packaging, and computer-aided design of wide bandgap based electronics. These emerging trends are described along with some early results indicating the additional improvements possible in power density. Operation at extreme temperatures also becomes more feasible.

Recent Progress in SiC and GaN Power Devices

Owing to low conduction/switching loss, high switching frequency and high operating temperature, wide bandgap SiC and GaN power devices are attractive for next-generation power electronic systems with increased efficiency and power density. Because of the superior performance, SiC and GaN power devices could be in position to compete with the mainstream Si power devices in a variety of applications, including household appliances, photovoltaic inverters, electric vehicles, data centers, etc. Despite numerous research efforts devoted to material growth, device fabrication and circuit demonstration in the past decades, wide bandgap power semiconductor devices are still confronted with challenges that need to be addressed through new structure design and advanced process development. In this talk, we will review recent progress in SiC and GaN power devices, including the following. (1) High-density of interface traps usually exist at the gate-oxide/SiC interface in SiC MOSFETs, presenting a critical challenge to the channel mobility and device stability. Interface engineering techniques for SiC MOSFETs (e.g. NO/N2O annealing, P incorporation, metal interfacial-layer, high-k dielectric deposition, etc.) and their impact on interface trap distribution as well as channel mobility will be discussed. (2) Long-term reliability of SiC MOSFETs with special focus on time-dependent dielectric breakdown (TDDB) will be presented. (3) We will introduce our latest progress in design and fabrication of super junction SiC devices, which can break the fundamental limit of the conventional unipolar devices. (4) In GaN-based lateral power devices, the 2DEG channel is an inherent normally-on channel, due to the spontaneous and piezoelectric polarization effects. To achieve fail-safe operation and simpler gate drive circuit, normally-off device technology including p-(Al)GaN cap, barrier recess and plasma treatment in the gate region will be introduced and compared. (5) Gate stack engineering techniques toward high-reliability GaN MIS-HEMTs/FETs will also be presented.

Thermal reliability of SiC device with Cu sintering die-attach processed at 250°C in N2 gas

Abstract Thermostable die-attach method alternative to soldering with Pb content is urgently required for utilizing wide-bandgap power semiconductors like SiC or GaN with a high maximum junction temperature TJ. Ag nano-or micro particle paste sintering attracts wide attention for this purpose, but Cu paste has also being developed mainly because of the cost pressure. Recently Cu sub-micron particle paste dispersed polyethylene Glycol solvent has demonstrated low temperature sintering at 250°C in N2 gas, resulting in high strength of die-attach. In the present study, the developed Cu paste die-attach has accepted to SiC Schottky barrier diode in TO-247 standard packaging with highly thermostable mold materials. The produced sample devices are subjected to high temperature storage tests at 250°C and thermal cycling tests between −50°C and 250°C. The device including the novel die-attach structure exhibited excellent thermal stability. Furthermore, power cycling tests has been conducted, and the results appear prospective for long life time and high reliability with high TJ exceeding 250°C. The presented study thus conclude that Cu submicron paste sintering is a promising candidate of power device die-attach usable for heavy industrial applications.

Influence of adatom migration on wrinkling morphologies of AlGaN/GaN micro-pyramids grown by selective MOVPE**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274039 and 61574173), the National Key Research and Development Program, China (Grant No. 2016YFB0400105), the International Science and Technology Collaboration Program of Guangdong Province, China (Grant No. 2013B051000041), the International Science and Technology Collaboration Program

GaN micro-pyramids with AlGaN capping layer are grown by selective metal–organic–vapor phase epitaxy (MOVPE). Compared with bare GaN micro-pyramids, AlGaN/GaN micro-pyramids show wrinkling morphologies at the bottom of the structure. The formation of those special morphologies is associated with the spontaneously formed AlGaN polycrystalline particles on the dielectric mask, owing to the much higher bond energy of Al–N than that of Ga–N. When the sizes of the polycrystalline particles are larger than 50 nm, the uniform source supply behavior is disturbed, thereby leading to unsymmetrical surface morphology. Analysis reveals that the scale of surface wrinkling is related to the migration length of Ga adatoms along the AlGaN facet. The migration properties of Al and Ga further affect the distribution of Al composition along the sidewalls, characterized by the μ-PL measurement.

(Invited) Wide Bandgap Analog and Mixed-Signal IC Design for Advanced Power Electronics

Economy and performance are benefits that come with high power density power electronics, just as in the case of VLSI electronics. High density power electronics require the heterogeneous integration of disparate technologies including power semiconductor devices, driver, protection and control circuitry, passives and voltage isolation techniques into single modules. Such integration activity was central to the Google Little Box Challenge competition conducted last year. One of the keys to advancing power electronic integration has been the commercial reality of wide bandgap power semiconductor devices made from silicon carbide and gallium nitride. The ability to design and manufacture wide bandgap integrated circuits as drivers, controllers, and protection circuitry allows them to be packaged in close proximity to the power device die to minimize parasitics that would adversely impact system performance. These impacts include excessive ringing, noise generation, power loss, and, potentially, self-destruction. The state of the art in wide bandgap analog and mixed-signal IC design will be described including example circuits such as amplifiers, data converters, controllers, and protection circuits and their integration into power electronic platforms. A synthesis tool under development for heterogeneous power circuit layout will be briefly described as a capstone to the application space descriptions. Advances in semiconductor device modeling of low-voltage SiC MOSFETs will be described as well.

Advanced aircraft power electronics systems — The impact of simulation, standards and wide band-gap devices

The rapid pace of change in the wide band gap (WBG) power semiconductor area has led to an explosion in potential uses for WBG devices in a huge variety of applications. The applications include automotive, aerospace and traction applications, as well as grid related or charging systems, with the potential to provide paradigm shifts in performance and efficiency over Silicon devices in current use today. Despite these exciting developments, however, there are still many outstanding challenges for both researchers and industry to solve before WBG technology becomes pervasive. In this paper we will explore some of these challenges and highlight the strengths of WBG devices, some of the specific issues for machine drives and develop some potential solutions for future developments in power electronics.

Standards in Power Electronics: Insight on handling current standards

Power electronics is a rapidly growing field, with an accelerating pace of technological change. This combination presents a challenge for industry professionals to stay up to date on the regulatory and standards requirements and the standards bodies (such as the IEEE) to ensure that the published standards are met. Standards are vitally important for confirming that products conform to the latest levels of functional interoperability, performance, safety, and reliability. This is essential at all levels from consumer products such as electric toothbrushes to cars and aircraft. The plethora of standards and regulatory bodies can lead to confusion, ignorance, and inadvertent problems that could be averted if designers and product engineers have a greater awareness of not only the standards required but a better idea of how to interpret and implement them. This article is intended to assist power electronics engineers in navigating these difficult and complex issues and to provide insight on how best to handle the use of standards. I use a number of commonly asked questions that practicing engineers have posed during my standards role within the IEEE Power Electronics Society (PELS). It is my hope that the answers are constructive and useful. In addition, I discuss significant activities aimed at establishing a road map for wide-bandgap (WBG) power semiconductors, which are not directly standards as such but are closely related to a number of standards activities.

Identification of the primary compensating defect level responsible for determining blocking voltage of vertical GaN power diodes

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (∼104 - 106 cm−2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at Ec-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be Nt = 3 × 1012, 2 × 1015, and 5 × 1014 cm−3, respectively. The Ec-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large VBD in the next-generation wide-bandgap power semiconductor devices. Thus, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.

Radiation resistance of wide-bandgap semiconductor power transistors

Radiation resistance of state-of-the-art commercial wide-bandgap power transistors, 1700 V 4H-SiC power MOSFETs and 200 V GaN HEMTs, to the total ionization dose was investigated. Transistors were irradiated with 4.5 MeV electrons with doses up to 2000 kGy. Electrical characteristics and introduced defects were characterized by current–voltage (I–V), capacitance–voltage (C–V), and deep level transient spectroscopy (DLTS) measurements. Results show that already low doses of 4.5 MeV electrons (>1 kGy) cause a significant decrease in threshold voltage of SiC MOSFETs due to embedding of the positive charge into the gate oxide. On the other hand, other parameters like the ON-state resistance are nearly unchanged up to the dose of 20 kGy. At 200 kGy, the threshold voltage returns back close to its original value, however, the ON-state resistance increases and transconductance is lowered. This effect is caused by radiation defects introduced into the low-doped drift region which decrease electron concentration and mobility. GaN HEMTs exhibit significantly higher radiation resistance. They keep within the datasheet specification up to doses of 2000 kGy. Absence of dielectric layer beneath the gate and high concentration of carriers in the two dimensional electron gas channel are the reasons of higher radiation resistance of GaN HEMTs. Their degradation then occurs at much higher doses due to electron mobility degradation.

(Invited) Current Topics in Electronic Devices Based on Wide Band-Gap Semiconductors for Power Applications and Energy Efficiency

Wide-bandgap power semiconductor devices offer enormous energy efficiency gains in a wide range of potential applications. As silicon-based semiconductors are fast approaching their performance limits for high power requirements, gallium nitride (GaN), silicon carbide (SiC), and diamond, with their superior electrical properties are likely candidates to replace silicon in the near future. Along with higher blocking voltages wide-bandgap semiconductors offer breakthrough relative circuit performance enabling low losses, high switching frequencies, and high temperature operation. The progress of the development of high voltage, high current wide-bandgap power switching devices in the APRA-E SWITCHES program is reviewed. The performance of various rectifiers and transistors, which have been demonstrated, is discussed. Material and processing challenges and reliability concerns for wide-bandgap power devices are also described. A glimpse into the future trends in device development and commercialization is offered.

(Invited) Current Topics in Electronic Devices Based on Wide Band-Gap Semiconductors for Power Applications and Energy Efficiency

Wide-bandgap power semiconductor devices offer enormous energy efficiency gains in a wide range of potential applications. As silicon-based semiconductors are fast approaching their performance limits for high power requirements, gallium nitride (GaN), silicon carbide (SiC), and diamond, with their superior electrical properties are likely candidates to replace silicon in the near future. Along with higher blocking voltages wide-bandgap semiconductors offer breakthrough relative circuit performance enabling low losses, high switching frequencies, and high temperature operation. The progress of the development of high voltage, high current wide-bandgap power switching devices in the ARPA-E SWITCHES program is reviewed. The performance of various rectifiers and transistors, which have been demonstrated, is discussed. Material and processing challenges and reliability concerns for wide-bandgap power devices are also described. A glimpse into the future trends in device development and commercialization is offered.

Integrated Microchannel Cooling for Power Electronic Modules

Abstract The power electronic module plays a key role in the power system by providing the needed physical support, electrical contact and insulation, and thermal pathway for power devices. Using wide bandgap power semiconductors in the power modules enables high-frequency and low-loss switching at relatively high temperatures for efficient power conversion. These advantages could lead to an increase in power-density for the power module as well as a reduction of cost, weight, and volume at the system level. However, the highly integrated power module requires advanced thermal management solutions for effective heat removal from the active chips to achieve high reliability. The evaluation of thermal performance for the power module is critical for its packaging design, because most of the heat generated by the semiconductors is dissipated through the module package. It is even more critical for the gallium nitride (GaN)-based power modules due to the lower thermal conductivity of the GaN material compared with that of silicon and silicon carbide. This paper provides a brief introduction of power modules in conventional packaging design and a review of several new packaging structures with advanced thermal management solutions. The direct-bonded-copper (DBC) substrate with integrated microchannel cooling designed for a new packaging structure is proposed for highly integrated power modules. In this design, the cooling microchannels are embedded inside the aluminum nitride (AlN) layer of the DBC substrate. In finite element analysis (FEA) simulation model of the new package, six high-voltage GaN transistors are arranged on the top surface of the DBC substrate to realize a three-phase inverter circuit. Three straight embedded microchannels with a cross-sectional area of 0.3 mm × 5 mm are located underneath the GaN devices. The average maximum temperature of the GaN devices in the new package is around 72 °C (50 W power loss applied on each die), which is about 16 °C lower than that in the traditional power module package. A thermal transfer coefficient of 2000 W/m2 K, which is equivalent to the liquid cooling condition, is applied on the bottom surface of the baseplate in the traditional package. Enhanced heat dissipation capability is demonstrated using this integrated microchannel cooling method. Further study will focus on the fabrication of a prototype and experimental testing.

Design of a Low Inductance Power Module Based on Low Temperature Co-fired Ceramic

Abstract Efficient, compact, and reliable power electronic modules are building blocks of modern day power electronic systems. In recent times, wide bandgap semiconductor devices, such as, silicon carbide (SiC) and gallium nitride (GaN), are widely investigated and used in the power electronic modules to realize power dense, highly efficient, and fast switching modules for various applications. For high power applications is it required to parallel and series several devices to achieve high current and high voltage specifications, which results in larger current conducting traces. One of the major obstacles in using these wideband gap power semiconductor devices are the internal module stray inductance that is associated with these current conducting traces. With increasing demand for higher switching frequency, the internal module parasitic inductance must be reduced to as minimum as possible in order to utilize the full potential of the wide bandgap devices. A multi-layer approach of low-temperature co-fired ceramic (LTCC) to package the wide bandgap devices is investigated. The multi-layer design freedom by using LTCC can be utilized to reduce the footprint of the overall power module, electrical interconnects, hence, reducing the package parasitic inductance. LTCC also facilitates high temperature operations and has a coefficient of thermal expansion matching with wide bandgap devices. In this paper, we report on a LTCC based power module design where LTCC is utilized as an isolation layer between the source and the drain of the power devices. A simulation based parasitic inductance analysis and electro-thermal-mechanical study is performed using ANSYS Workbench Tools to investigate the feasibility of this LTCC based design.

Special issue on wide-bandgap semiconductor power electronics

Special issue on wide-bandgap semiconductor power electronics

A High Performance Power Package for Wide Bandgap Semiconductors Using Novel Wire Bondless Power Interconnections

Silicon carbide (SiC) wide bandgap semiconductor power device technologies offer improved electrical and thermal performance over silicon in high performance power electronic applications, such as hybrid or fully electric vehicles, aerospace, solar inverters, and advanced military systems. However, current packaging limitations make it difficult to operate these devices to their full potential. One such limitation includes die interconnections, which are traditionally made using large diameter aluminum wire bonds. This discussion introduces an innovative wire bondless interconnect technique for power packaging called PowerStep. This approach includes a precision etched metal tab with raised regions matching the size and location of device terminals and trenches to accommodate critical features on devices such as passivated surfaces. The tab offers a low profile, low inductance, low resistance, high electrical and thermal conductivity, and mechanically rugged interconnection solution. PowerStep has been implemented to facilitate a single-step interconnection method for a 600 to 1700 V SiC electronics package, replacing wire bonds which are connected one at a time. In addition, a key element of this package is the absence of a baseplate, resulting in lower weight, volume, and cost, as well as reduced manufacturing complexity. The electrical, thermal, and mechanical characteristics of PowerStep interconnections are analyzed and compared to conventional aluminum wire bonds to demonstrate the advantages of wire bondless interconnections coupled with wide bandgap devices. The low parasitics and junction-to-case thermal resistance of the package combined with PowerStep interconnects capture the high performance of SiC for power applications.

(Invited) Baseplate Materials for Securing Reliability of Wide Band Gap Power Semiconductor Module Operating at High Temperatures

Baseplate material properties for realizing high temperature resistant power modules are investigated. Power modules should have a tolerance for heavy thermal cycle load and retain their original performance. Therefore, it is important to improve the reliability of the heat dissipation path, which is the soldered joint between the baseplate and ceramic substrate. This study aimed to improve reliability by appropriately selecting the baseplate material and relaxing the thermal stress in the soldered joint. We conducted a series of verification tests using four types of baseplate material. Our results clarified that a smaller mismatch of coefficient of thermal expansion (CTE) between the baseplate and ceramic substrate contributes to an improvement in reliability, especially in case of W–Cu as the baseplate material. Conversely, the results also clarified that baseplate plastic deformation contributes to an improvement in reliability even if the CTE mismatch is large, in the case of Al.