Experimental Detection and Numerical Validation of Different Failure Mechanisms in IGBTs During Unclamped Inductive Switching

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_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.

I. Introduction

The insulated-gate bipolar transistor (IGBT) devices used as a power switch for medium-voltage applications (800–1200 V) must conjugate the performance requirements of low on-resistance and fast switching, with the mandatory target of high device ruggedness. As a matter of fact, device ruggedness is critical for medium-voltage applications as the power load is inductive in nature. This holds particularly true if the IGBT is used in inverter topologies for hybrid electric vehicle purposes where device robustness to avalanche is a mandatory requirement for a reliable operation. Unclamped inductive switching (UIS) is a high-stress test used to determine the maximum amount of avalanche energy a device can handle by forcing a charged unclamped inductor to discharge through the device under test (DUT). This test is commonly used in the automotive industry to screen weak devices prior to their usage in the chosen application. During UIS test, the device discharges the energy stored in the inductive load being forced in the breakdown condition. The collector voltage is therefore equal to the breakdown voltage (BV) of the DUT.