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Thermal instability presented by some high current power MOS has been shown to limit significantly the SOA capability. In this paper we present a new analytical model to explain this type of instability in transient operation, based on an analytical formulation for both the positive temperature coefficient of the drain current and for the thermal resistance. The model is capable to predict the onset of thermal instability for a given device structure and lay-out, and can be used both to define the allowed SOA of the device and as a design guide to design more rugged devices.

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The physics of the different failure modes that limit the maximum avalanche capability during unclamped inductive switching (UIS) in punchthrough (PT) and not PT (NPT) insulated-gate bipolar transistor (IGBT) structures is analyzed in this paper. Both 3-D electrothermal numerical simulations and experimental evaluations support the theoretical analysis. Experimental results for UIS test show that, at low time duration (or inductance value) of the test, the UIS limit moves from energy limitation to current limitation. While the energy limitation is well known, the current-limited failures are less studied. In this paper, the current limit for UIS test is analyzed in detail, and the cause is attributed to a filamentary current conduction due to the presence of a negative differential resistance (NDR) region in the $I_{C}$– $V_{rm CE}$ curve in breakdown. The filamentary current conduction locally increases the current density causing early device latch-up and possible device failure at a current much lower than the one dictated by energy limitations. The physical parameters that affect both the onset of NDR region and the failure current are discussed for both an NPT trench IGBT structure with a local lifetime control and a PT trench IGBT structure with a field-stop layer

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This paper reports on the results of a study on electro-thermal instability induced in multi-cellular TrenchIGBTs in avalanche condition. Experimental measurements, made on T-IGBTs, show possible inhomogeneous current distribution under Unclamped Inductive Switching (UIS) confirmed by transient infrared thermography measurements. Together with this, an analytical modeling of avalanche behavior has been included in a compact electro-thermal simulator to study the interaction between a large numbers of elementary cells of TIGBTs forced in avalanche condition. Electro-thermal simulations qualitatively replicate the possible inhomogeneous operation observed experimentally. Finally a possible theoretical interpretation of the instability in avalanche condition for T-IGBT is given.

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