Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

A Survey of Wide Bandgap Power Semiconductor Devices

In this paper, we present a comprehensive reviewand discussion of the state-of-the-art device technology and application development of GaN-on-Si power electronics. Several device technologies for realizing normally off operation that is highly desirable for power switching applications are presented. In addition, the examples of circuit applications that can greatly benefit from the superior performance of GaN power devices are demonstrated. Comparisonwith other competingpower device technology, such as Si superjunction-MOSFET and SiC MOSFET, is also presented and analyzed. Critical issues for commercialization of GaN-on-Si power devices are discussed with regard to cost, reliability, and ease of use.

GaN-on-Si Power Technology: Devices and Applications

Advances in power electronics enable efficient and flexible processing of electric power in the application of renewable energy sources, electric vehicles, adjustable-speed drives, etc. More and more efforts are devoted to better power electronic systems in terms of reliability to ensure high availability, long lifetime, sufficient robustness, low maintenance cost and low cost of energy. However, the reliability predictions are still dominantly according to outdated models and terms, such as MIL-HDBK-217H handbook models, Mean-Time-To-Failure (MTTF), and Mean-Time-Between-Failures (MTBF). A collection of methodologies based on Physics-of-Failure (PoF) approach and mission profile analysis are presented in this paper to perform reliability-oriented design of power electronic systems. The corresponding design procedures and reliability prediction models are provided. Further on, a case study on a 2.3 MW wind power converter is discussed with emphasis on the reliability critical components IGBTs. Different aspects of improving the reliability of the power converter are mapped. Finally, the challenges and opportunities to achieve more reliable power electronic systems are addressed.

Design for reliability of power electronic systems

In this paper, we report on 1.2-kV-class vertical GaN-based trench metal–oxide–semiconductor field-effect transistors (MOSFETs) on a free-standing GaN substrate with a low specific on-resistance. A redesigned epitaxial layer structure following our previous work with a regular hexagonal trench gate layout enables us to reduce the specific on-resistance to as low as 1.8 mΩcm2 while obtaining a sufficient blocking voltage for 1.2-kV-class operation. Normally-off operation with a threshold voltage of 3.5 V is also demonstrated. To the best of our knowledge, this is the first report on vertical GaN-based MOSFETs with a specific on-resistance of less than 2 mΩcm2.

http://iopscience.iop.org/article/10.7567/APEX.8.054101/pdf

1.8 mΩ·cm2 vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate for 1.2-kV-class operation

Many power switching devices are used in hybrid vehicles (HVs) and electric vehicles (EVs). To improve the efficiency of HVs and EVs, better performance characteristics than those of Si power devices, for example, lower on-resistance, higher speed, higher operation temperature, are required for the power devices. GaN power devices are promising candidates for satisfying the requirements. A lateral GaN power device with a blocking voltage of 600 V and a vertical GaN power device with a blocking voltage of 1200 V are suitable for medium power applications for sub systems and high-power applications for the drive of main motors, respectively. Power device applications in HVs and EVs and the current status of the GaN power device are presented. The reliability of the GaN power device is also discussed.

https://iopscience.iop.org/article/10.7567/JJAP.53.100210/pdf

Recent progress of GaN power devices for automotive applications

Normally-off GaN transistors for power applications in p-type GaN gate technology with a modified carbon-doped GaN buffer are presented. A combination of an AlGaN back-barrier with the carbon-doped buffer prevents early off-state punch-through. Simultaneously, the on-state resistance could be kept low and the threshold voltage with 1.1 V high enough for secure normally-off operation. 1000 V breakdown strength has been obtained for devices with 6 μm gate-drain spacing. The resulting breakdown scaling slope is 170 V/μm gate-drain distance. The on-state resistance is 7.4 Ωmm. The resulting V Br -to-R ON A ratio (1000 V, 0.62 mΩcm2) is beyond so far reported ratios for normally-off GaN transistors. Modifications of the p-type GaN layer have shown to additionally increase the threshold voltage by 0.4 V without paying a price in the on-state resistance of the device.

Normally-off high-voltage p-GaN gate GaN HFET with carbon-doped buffer

Reliability issues of GaN based high voltage power devices

Switching experiments with normally-off GaN-HFETs using a carbon-doped GaN buffer or an AlGaN buffer showed very different magnitudes of increased dynamic on-state resistance. The dynamic on-state resistance is analyzed for variations in buffer composition and set into relation to the buffer voltage-blocking strength. Also, the impact of p-GaN gate normally-off and Schottky-gate normally-on device technologies on the dispersion is studied. It is concluded that a buffer with less trap sites and lower breakdown strength is more favorable for high-voltage switching than a buffer with incorporated acceptors to increase the buffer breakdown strength.

Impact of buffer composition on the dynamic on-state resistance of high-voltage AlGaN/GaN HFETs

An example of GaN buffer structure optimization in AlGaN/GaN heterojunction field-effect transistors is demonstrated. Transistors fabricated on four epitaxial structures with buffer consisting of unintentionally doped GaN channel (35 nm or 100 nm) and carbon doped GaN:C layers (∼1 × 1018 cm−3 or ∼1 × 1017 cm−3) are compared. As the criteria for optimization off-state breakdown voltage (Vbr) and drain current dispersion are used. The observed trade-off between the two parameters and dependency of Vbr on the carbon concentration and on the channel thickness are explained by a potential barrier formed due to GaN:C part of the buffer.

E. Cho

Papers

Normally-off high-voltage p-GaN gate GaN HFET with carbon-doped buffer

Reliability issues of GaN based high voltage power devices

Impact of buffer composition on the dynamic on-state resistance of high-voltage AlGaN/GaN HFETs

Off-state breakdown and dispersion optimization in AlGaN/GaN heterojunction field-effect transistors utilizing carbon doped buffer

Impact of AlN nucleation layer on strain in GaN grown on 4H-SiC substrates