Observation of photoemission behaviour during avalanche breakdown of insulated gate bipolar transistor with defect in the metal contact

Abstract

Under unclamped inductive switching (UIS) condition, there is non-uniformity in the intra-chip distribution of avalanche breakdown current in insulated gate bipolar transistors (IGBTs). The “current filaments”, which are concentrations of current caused by the parasitic PNP transistor, move through the chip and are thought to be related to failures. During the avalanche breakdown, the charge bremsstrahlung generates light over a wide wavelength range. We have previously reported that we constructed an observation system using a photomultiplier tube (PMT) and observed the avalanche breakdown photoemission with nanosecond time resolution. In this study, we used the same observation system to observe the photoemission behaviour of the samples in which the metal electrode was partially removed with a FIB (focused ion beam) to create a defective contact area. In the sample with the pseudo-defect, (a) there was an abnormal emission peak that did not correspond to the avalanche current change, (b) the randomness of the behaviour for each single pulse became extremely low, and (c) the pseudo-defect itself did not trap the current filament. These results may explain the tendency of IGBTs to have a higher probability of failure during repetitive avalanche breakdowns if they contain some defects.