On the Origin of Thermal Runaway in a Trench Power MOSFET

In this paper, we investigate the origin of thermal runaway in the trench power MOSFET of a modern smart power IC technology. Experimental data on the temperature rise during power pulses show that the onset of thermal runaway depends on the biasing condition even if the power pulses have equal power dissipation. The beginning of thermal runaway in this work is denoted by the inflection point in the measured temperature data. For the experimental data points, the onset varies from 340 °C to 520 °C. Comparison of these experimental data with an analysis based on the stability factor shows very good agreement. The stability factor analysis demonstrates that, above the temperature compensation point (TCP), the driving force for thermal runaway is the thermally generated leakage current of the parasitic n-p-n bipolar transistor. The decrease of mobility and, hence, the MOS channel current above the TCP stabilizes the power MOSFET. In contrast, below the TCP, both the increase of the MOS channel current and the parasitic n-p-n bipolar transistor leakage current with temperature contribute to the thermal runaway.