Failure mechanism and improvement potential of IGBT´s short circuit operation

In this work we will focus on the short circuit destruction of IGBTs after turn-off. Electro-thermal device simulations and experimental verifications have been carried out to study the failure mechanism and identify the improvement potential. Heat diffusion to the front and back side chip surfaces after the short circuit pulse causes temperature rises at the pn-junctions which in turn inject thermally induced leakage currents into the drift region. They can lead to self-heating up to device destruction if the heat transport out of the chip is limited by a poor thermal setup. The combination of a significant heat capacity at the chip front side with a significant reduction of the thermal resistance of the solder layer has been found to be most efficient for increasing the short circuit capability. 1200V trench field-stop IGBTs with a thick front side metallization - made of copper instead of aluminum - have been fabricated and have been attached to the DBC by a newly developed diffusion soldering process. This way the critical short circuit energy before destruction has been increased by 80% which has also been predicted by simulation. In contrast, attaching a separate heat capacity by soldering a molybdenum cube to the chip front side did not yield the expected improvement. Active IGBT cells which could be not covered by the heat sink failed at the original energy level.