Hiromichi Ohashi

High breakdown voltage AlGaN-GaN power-HEMT design and high current density switching behavior

AlGaN-GaN power high-electron mobility transistors (HEMTs) with 600-V breakdown voltage are fabricated and demonstrated as switching power devices for motor drive and power supply applications. The fabricated power HEMT realized the high breakdown voltage by optimized field plate technique and the low on-state resistance of 3.3 m/spl Omega/cm/sup 2/, which is 20 times lower than that or silicon MOSFETs, thanks to the high critical field of GaN material and the high mobility in 2DEG channel. The fabricated devices also demonstrated the high current density switching of 850 A/cm/sup 2/ turn-off. These results show that AlGaN-GaN power-HEMTs are one of the most promising candidates for future switching power device for power electronics applications.

Oscillation effects in IGBT's related to negative capacitance phenomena

Insulated gate bipolar transistors (IGBTs) are inherently unstable at high collector voltages due to negative gate capacitance values. We investigate IGBT gate voltage oscillations by experiment and through computer simulation. In addition, we show that under certain gate circuit conditions, gate voltage oscillations can lead to already observed collector current imbalance effects.

Diamond Schottky-pn diode with high forward current density and fast switching operation

We fabricated a diamond diode, namely a Schottky-pn diode (SPND), which is composed of a fully depleted n-type active layer sandwiched between a highly doped p-type layer and a Schottky metal layer. The SPND has superior characteristics that overcome the weak points of both a Schottky barrier diode and a pn diode. That is, the SPND showed high current density (over 4000 A/cm2) with low specific resistance (0.4 mΩ cm2) at a forward bias of 6 V while maintaining a high rectification ratio of ∼1010. Moreover, the SPND showed extremely fast turn-off speed of nanosecond order.

High-efficiency high-power dc-dc converter for energy and space saving of power-supply system in a data center

This paper presents the power density developments of high-power isolated dc-dc converter to be utilized in a higher voltage direct-current (HVDC) power-supply system of a data center. This technique results in a prototype of dc-dc converter unit which is smaller by factor of ten when compared with the conventional one, and the area of power-supply system becomes much smaller than before. Consequently, the cooling power for this power-supply system is saved and both the energy and the space saving are available. In order to realize a prototype of the high-power isolated dc-dc converter, the switching power devices composed of a hybrid pair of Si-CoolMOS and SiC-SBD are utilized and operated in a driving pattern of hard switching with a frequency of 200kHz. As a result, an isolated dc-dc converter with the input/output voltage of 400V/400V, a rating power of 5kW, and a high power density of 10W/cm3 has been fabricated. The experimental confirmation was reported and the surge problems due to diode recovery difficulties mentioned before was solved.

SiC-SIT Circuit Breakers With Controllable Interruption Voltage for 400-V DC Distribution Systems

400-V direct current (400-V dc) distribution systems are promising candidate of highly efficient and reliable distribution systems for data centers. To realize the 400-V dc distribution systems, development of methods for high-speed overcurrent protection is one of the important issues. In this paper, as a solution to this problem, a semiconductor dc circuit breaker using SiC static induction transistors (SiC-SITs) is investigated. The SiC-SITs has extremely low on-state resistance and very large safe operating area (SOA). These properties are attractive in the application of the circuit breakers for 400-V dc distribution systems. A novel control method of the gate voltage waveform to reduce the transient overvoltage and resonance during the interruption process is proposed. The effectiveness of the proposed method is confirmed by actual operation tests employing an experimental prototype of the dc distribution system. All of the result is confirmed by the fabricated SiC-SIT circuit breaker prototype.

GaN Power Transistor Modeling for High-Speed Converter Circuit Design

A circuit simulator has been developed to design power losses of high-frequency power converters using GaN-based heterojunction field-effect transistors (GaN-HFETs). The simulator is based on a high-accuracy equivalent model of GaN-HFETs with peculiar device physics and high-speed loss calculation methods. The simulated power losses were consistent with measured results in dc-dc converters constructed by a GaN-HFET and a SiC Schottky diode with more than 93% accuracy. By utilizing the developed simulator, key requirements in heat-dissipation technologies, circuit parasitic inductances, and gate-drive technologies for next-generation converters are discussed.

High breakdown voltage AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor with TiO2/SiN gate insulator

We report the fabrication of an AlGaN/GaN high-electron-mobility transistor (HEMT) with a high breakdown voltage by employing a metal–insulator–semiconductor (MIS) gate structure using TiO2/SiN insulators. We employed the TiO2/SiN gate insulator for the first time in a multilayered insulator structure MIS-HEMT. The gate leakage current was significantly reduced by employing the MIS structure, and the breakdown voltage characteristics of the fabricated MIS-HEMTs were 1.1 kV with an on-resistance of 15 mΩ cm2 for a gate-drain length of Lgd = 28 µm. The current collapse in the TiO2/SiN MIS-HEMT has been improved by employing a thin SiN film under the TiO2 insulator. AlGaN/GaN MIS-HEMTs are promising not only for high-speed applications but also for high-power switching applications.

An investigation of voltage balancing circuit for DC capacitors in diode-clamped multilevel inverters to realize high output power density converters

Output power density of power converters has been increasing. At the same time, the power converters with lower harmonics and lower electromagnetic interference are required in many applications. To satisfy these requirements, the volume of filters and heat sinks should be reduced. As a solution to realize higher output power density and lower harmonics, diode-clamped multilevel inverters are investigated. In this case, the voltage balancing circuit must be connected to the DC capacitors because the DC voltages tend to unbalance when the number of the output level exceeds three. Therefore, the total volume of the converters including the voltage balancing circuit should be investigated. In this paper, resonant switched capacitor converter is treated as a promising solution to realize the voltage balancing circuit with smaller volume. The operating characteristics and estimated volume are clarified analytically. Finally, the applicability of the voltage balancing circuit is confirmed by the experimental results.

Study on advanced power device performance under real circuit conditions with an exact power loss simulator

Power losses of 700V 2.7mOmegacm2 SiC-MOSFETs including influences of circuit stray inductances have been investigated with an originally developed circuit power loss simulator. The device parameters of the SiC-MOSFETs for the power loss calculation are extracted from a SiC-IEMOSFET fabricated at AIST PERC. The power losses of three types of chip areas are investigated and compared to power loss of a CoolMOS. The switching loss energies of the SiC-MOSFETs are larger than that of the CoolMOS in the same circuit conditions. The maximum switching frequencies of the SiC-MOSFETs are 1/2 ~ 2/3 times lower than that of the CoolMOS. Based on total circuit power loss calculation results, heatsink volumes for a half bridge type DC-DC converter at 200kHz operation are estimated. In the case of the SiC- MOSFET3(smallest chip size)/SiC-SBD pair, the heatsink volume can be reduced to about 45% compared to the CoolMOS/SiC-SBD pair due to the Tj=200degC operation and the lower circuit power loss.

New collector design concept for 4.5 kV injection enhanced gate transistor (IEGT)

In this paper, we propose a new collector design concept for high voltage IGBTs/IEGTs, which is composed of a low injection efficiency p-emitter and low transport factor n-buffer. The n-buffer with low transport factor provides decreasing hole injection from p-emitter to n-base to improve the trade-off relation between collector-emitter saturation voltage and turn-off loss. Besides, the n-buffer with low transport factor does not prevent hole injection from p-emitter to n-base at the device turn-on period. As the low transport n-buffer is combined with a low efficiency p-emitter, total switching loss including turn-off loss and turn-on loss decreases. The new collector design concept was applied to 4.5 kV IEGTs, and the fabricated device has demonstrated a reduction of total switching loss by 11% compared with that of the device having only low injection efficiency p-emitter. The fabricated device with the new collector design concept also has excellent turn-off capability.