Tomoyuki Shoji

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.

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.

Theoretical analysis of short-circuit capability of SiC power MOSFETs

The short-circuit capability of Si power devices, defined in terms of critical energy density, is the product of the heat capacity in the heat generation region and the increase in temperature. However, for SiC power devices, the heat generation region is significantly smaller than that for Si power devices, because the drift-region thickness is about 10 times less in SiC power devices. Therefore, the formulae used for Si devices are not directly applicable to SiC devices. In this study, analytical formulae are derived for the short-circuit capability of a SiC power device and its dependence on the ambient temperature and the thickness of the n− drift region, on the basis of the thermal diffusion equation. The calculated results are found to be in good agreement with those of direct measurements.

Experimental and simulation studies of neutron-induced single-event burnout in SiC power diodes

Neutron-induced single-event burnouts (SEBs) of silicon carbide (SiC) power diodes have been investigated by white neutron irradiation experiments and transient device simulations. It was confirmed that a rapid increase in lattice temperature leads to formation of crown-shaped aluminum and cracks inside the device owing to expansion stress when the maximum lattice temperature reaches the sublimation temperature. SEB device simulation indicated that the peak lattice temperature is located in the vicinity of the n?/n+ interface and anode contact, and that the positions correspond to a hammock-like electric field distribution caused by the space charge effect. Moreover, the locations of the simulated peak lattice temperature agree closely with the positions of the observed destruction traces. Furthermore, it was theoretically demonstrated that the period of temperature increase of a SiC power device is two orders of magnitude less than that of a Si power device, using a thermal diffusion equation.

Observation and Analysis of Neutron-Induced Single-Event Burnout in Silicon Power Diodes

Annular microvoids formed by neutron-induced single-event burnout (SEB) in Si power diodes were observed by a slice-and-view technique. The axial symmetry of damage region reflects the spatially isotropic thermal diffusion that occurred. Analytical formulas for the local rise in temperature during SEB were derived from the thermal diffusion equation. The local temperature was found to increase in direct proportion to the deposited energy, which was expressed as the time integration of the product of the applied voltage and the SEB current. This current is the result of charges generated by recoil ions and subsequent current-induced avalanche. The diameter of the damage region was estimated using the analytical formulas and the energy associated with Joule heating, which was calculated by technology computer-aided design device simulations, and was found to be comparable in size to the observed annular voids. The SEB current density was also calculated based on the simulated SEB current and the size of the damage region.

Triggering Mechanism for Neutron Induced Single-Event Burnout in Power Devices

Cosmic ray neutrons can trigger catastrophic failures in power devices. It has been reported that parasitic transistor action causes single-event burnout (SEB) in power metal–oxide–semiconductor field-effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs). However, power diodes do not have an inherent parasitic transistor. In this paper, we describe the mechanism triggering SEB in power diodes for the first time using transient device simulation. Initially, generated electron–hole pairs created by incident recoil ions generate transient current, which increases the electron density in the vicinity of the n-/n+ boundary. The space charge effect of the carriers leads to an increase in the strength of the electric field at the n-/n+ boundary. Finally, the onset of impact ionization at the n-/n+ boundary can trigger SEB. Furthermore, this failure is closely related to diode secondary breakdown. It was clarified that the impact ionization at the n-/n+ boundary is a key point of the mechanism triggering SEB in power devices.

Analysis of neutron-induced single-event burnout in SiC power MOSFETs

Abstract The triggering mechanism of single-event burnout (SEB) in SiC power MOSFETs was studied by white neutron irradiation experiments and device simulations. Electron–hole pairs generated along a recoil ion trajectory resulted in a highly localized SEB current. This dynamic current led to an increase in the electron density in the vicinity of the n − /n + interface, which resulted in a shift in the peak electric field strength. Finally, a local short-circuit occurred between the drain and source electrodes by punch-through of the electric field in the n + source diffusion region. A cross-sectional view of the SEB damage showed that melting of the SiC occurred and cracks were formed in the n − drift region due to the highly localized SEB current. This indicates that the maximum lattice temperature reached the sublimation temperature of SiC. The location of the simulated peak lattice temperature agreed closely with the position of the observed SEB damage. This demonstrated that the main mechanism triggering SEB in SiC power MOSFETs is not parasitic npn-transistor action, but a shift in the peak electric field and the punch-through in the n + source diffusion region, similar to the case for SiC power diodes.

Neutron Induced Single-Event Burnout in SiC Power Diode

Neutron induced single-event burnouts (SEBs) of silicon carbide (SiC) power diodes have been investigated by white-neutron irradiation experiments and transient device simulations. It was confirmed that a rapid increase in lattice temperature leads to the formation of a crown-shaped aluminum and cracks inside the device due to internal stress when the local lattice temperature reaches the sublimation temperature. SEB device simulation indicated that the peak lattice temperature is located in the vicinity of the n/n boundary and p/n junction with high electric field strength. Moreover, the simulated peak lattice temperature corresponded to the positions of the observed destruction traces. From these facts, SEBs of SiC diodes can be characterized by lattice temperature increase up to the sublimation temperature because of the high heat generation density.