Improving the Short-Circuit Reliability in IGBTs: How to Mitigate Oscillations

In this paper, the oscillation mechanism limiting the ruggedness of insulated gate bipolar transistors (IGBTs) is investigated through both circuit and device analysis. The work presented here is based on a time-domain approach for two different IGBT cell structures (i.e., trench-gate and planar), illustrating the two-dimensional effects during one oscillation cycle. It has been found that the gate capacitance varies according to the strength of the electric field near the emitter, which in turn leads to charge-storage effects associated with low carrier velocity. For the first time, it has been discovered that a parametric oscillation takes place during the short circuit in IGBTs, whose time-varying element is the Miller capacitance, which is involved in the amplification mechanism. This hypothesis has been validated through simulations and its mitigation is possible by increasing the electric field at the emitter of the IGBT with the purpose of counteracting the Kirk effect.