Masayasu Ishiko

Investigations on current filamentation of IGBTs under undamped inductive switching conditions

We have investigated current filamentation of IGBTs occurring under UIS (undamped inductive switching) conditions, by using electro-thermal device simulations. In this paper, we present that the formation of a current filament inevitably takes place even if the device active region include no weak spots. In addition, it is clarified that the current filament travels inside the active region with Joule self-heating, and the filament pinning due to parasitic bipolar action at the weak spot leads to lowering UIS capability.

Mechanical stress dependence of power device electrical characteristics

This paper describes how mechanical stress affects the electrical characteristics of a power device, depending on the surface structure of the device or the device type. Experimental results show that devices in which the current flow direction is vertical to the substrate, such as trench structure devices, are affected the least by mechanical stress

Neutron induced single-event burnout of IGBT

Cosmic-ray neutrons can trigger a single-event burnout (SEB), which is a catastrophic failure mode in power semiconductor devices. It was found experimentally that the incident neutron induced SEB failure rate increases as a function of the applied collector voltage in an insulated gate bipolar transistor (IGBT). Moreover, the failure rate increased sharply with an increase in the applied collector voltage when the voltage exceeded a certain threshold value. Transient device simulation showed that the onset of impact ionization at the n− drift/n+ buffer junction (nn+ junction) can trigger turning-on of the inherent parasitic thyristor, and then SEB subsequently occurs. In addition, it was analytically derived that reducing the current gain of the parasitic transistor was effective in increasing the SEB threshold voltage. Furthermore, ‘white’ neutron-irradiation experiments demonstrated that suppressing the inherent parasitic thyristor action leads to an improvement of the SEB threshold voltage.

Effects of uni-axial mechanical stress on IGBT characteristics

This paper describes the impacts of mechanical stress on vertical power devices. The stress dependence of the DC characteristics of trench insulated gate bipolar transistors (IGBTs) was measured. The experimental results could be reproduced by the device simulation, which included stress dependence models of the carrier mobility and the band gap. We found that the stress dependence of the on-state voltage mainly arose from the MOSFET portion of the IGBT. Using the device simulation, we estimated the effects of mechanical stress on the surge voltage and the saturation current, which give us the important information for designing a power module with higher ruggedness.

A 600 V 200 A low loss high current density trench IGBT for hybrid vehicles

We developed a new low loss high destruction immunity 600 V 200 A trench IGBT for hybrid vehicles. In order to secure high destruction immunity at high current density use caused by chip shrink, we designed a process and a cell on the basis of results of both simulation and 2.5 mm square TEGs (Test Element Groups) experimental results. A 4 um trench pitch, punch-through wafer structure, and local lifetime technology are employed. As a result, the developed IGBT shows an on-state voltage drop of 1.55 V, a fall time of 800 ns, an avalanche capability of over 1 J (at R.T.), and a short circuit immunity of over 40 us. The IGBT is fitted with both a temperature sensor and a current sensor in order to avoid chip failure.

A normally-off bipolar mode static induction transistor (BSIT) with high current gains

The DC current gain of a normally-off bipolar mode static induction transistor with surface-gate structure can be improved by the reduction of the source- contact-area. To explain the current gain increase, a simple device model was introduced. It was found from this model that the DC current gains were in proportion to the inversion of the source-contact-area. The model is able to explain the experimental results well. In addition, the adequacy of the model was verified qualitatively by computer simulation. Based on the model, a high power BSIT module with high current gains has been developed.

Assessment of the Trench IGBT reliability: low temperature experimental characterization

The purpose of this study is an assessment of the Trench IGBT reliability at low temperature under static and dynamic operations by the aim of intensive measurements. The analysis of the Trench IGBT behaviour in these conditions is dedicated to the HEV applications. One question can be raised in case of the use of HEV in countries where during winter the temperature drops down -50°C or less: are Trench IGBT strongly affected by the low temperature environment? In this paper, we present experimental results under various test conditions (temperature, gate resistance, voltage and current) to give an understanding of the device behaviour by focusing on the device current and voltage waveforms and the power losses.

Electrical Properties of Metal?Insulator?Semiconductor Capacitors on Freestanding GaN Substrate

The electrical properties of GaN metal–insulator–semiconductor (MIS) capacitors with as-grown SiO2, annealed SiO2, and SiNx insulators were investigated by capacitance–voltage (C–V), current–voltage (I–V), and time-dependent dielectric breakdown (TDDB) measurements. The MIS capacitor with the SiNx insulator was determined to have a lower interface trap density of 1×1011 cm-2 eV-1 than the MIS capacitors with the as-grown and annealed SiO2 insulators. In addition, the dielectric lifetime of the SiNx insulator was extrapolated to be more than 20 years at an even high voltage at room temperature. These results indicate that the high insulator/GaN interface quality and high reliability required for high-performance power devices can be achieved by the deposition of the SiNx insulator on GaN.

Investigation of correlation between device structures and switching losses of IGBTs

This study investigated the correlation between the device structures and the switching power dissipation by using an index, the ratio of feedback capacitance to input capacitance. We point out that a wide-cell-pitch and an injection enhanced structure have inherently higher switching power dissipation under the fitted dV/dt and dI/dt condition because of their large index.

Production of Metastable Defects in n-Type Silicon by Hydrogen Implantation at 88 K

We report on the first observation of metastable defects in n-type silicon implanted with 90 keV hydrogen ions at 88 K at a dose of 2×1010 cm-2. Schottky contacts were fabricated on the implanted surfaces to study room-temperature stable defects by deep-level transient spectroscopy. After reverse-bias cooling, three new peaks (Ec–0.29, 0.41, 0.55 eV) appear in addition to well-known defects. Metastable defects are hydrogen-related and their production rates are higher in the 88 K implantation than in the room-temperature implantation.