Using the chip as a temperature sensor — The influence of steep lateral temperature gradients on the Vce(T)-measurement

During operation steep lateral temperature gradients evolve in IGBT power semiconductor chips. The influence of these lateral gradients on the measurement of the virtual junction temperature by means of the widely used V_CE(T)-method was investigated. In particular we address the question, how the obtained single temperature value is connected to the temperature distribution of the chip. A combination of electrical and thermal measurements together with thermal simulations was performed to understand the implicit averaging mechanisms of the V_CE(T)-measurement. It is found that the lateral temperature gradient in the chip results in an inhomogeneous sense current distribution during the measurement. This current distribution is responsible for the formation of the measurement value and its corresponding temperature T*. A comparison of experimental and simulation results shows that for currently existing IGBTs, T* corresponds to the area-weighted average of the chip´s active area. The maximum imbalance in sense current density during the V_CE(T)-measurement was determined to be 150% and 50% of the average current density for the central and the corner parts of the chip, respectively. Furthermore, the temporal evolution of the temperature profile and its influence on the thermal impedance measurement are discussed. It is shown that the temperature at the chip center evolves with a smaller thermal time constant (i.e. faster evolution) than at the chip corners.