Avalanche breakdown evolution under hot-carrier stress: a new microscopic simulation approach applied to a vertical power MOSFET

During avalanche breakdown, hot carriers are generated due to impact ionization. In a MOSFET, some of these hot carriers impinge on the field oxide and can be captured in the oxide. Over time, this leads to an accumulation of fixed charges at the Si/SiO _2 interface and consequently to a degradation of device characteristics. Among those characteristics, the drift of the avalanche breakdown voltage is one of the most important aspects of device reliability. In this study, we present the first microscopic simulation of the oxide charge generation due to hot carriers and their field-driven recombination under avalanche breakdown conditions in a vertical power MOSFET. The distribution functions of electrons and holes in the device are calculated by solving their coupled Boltzmann equations. Based on the hot-carrier distribution functions, the injection rates of hot electrons and holes into the field oxide are evaluated. The resulting drift of the avalanche breakdown voltage in this simulation is in good agreement with experimental results. The underlying device internal processes are discussed, and the gained insight might help to improve the reliability of power transistors.