Thermal Runaway Robustness of SiC VJFETs

One of the key advantage of SiC power devices over their Si counterparts is their ability to operate at higher temperature (in theory up to 1000 K for a 1000 V-rated SiC device as compared to less that 500 K for a comparable Si device [1]). Practical tests have already demonstrated operation at elevated temperature. For example, in [2], SiC JFET and diode operate at 450°C (723 K) ambient temperature, although with considerable de-rating in saturation current and blocking voltage. The high-junction-temperature uses of power devices can be divided in two sets of applications: operation in harsh environment (aerospace, oil exploration...), and operation with reduced cooling (small heatsink, confined system...). An example of this latter application is given in [3], with a converter designed to operate at an ambient temperature of 150°C with a junction temperature of its power devices as high as 250°C. This large drop between the junction and ambient temperatures allows for the use of a less sophisticated cooling system. However, it has been demonstrated in [4] that the high temperature advantage of SiC might be limited because of thermal runaway issues. In a recent article [5], we studied this phenomenon on SiC diodes, and showed that purely unipolar diodes are prone to thermal runaway, whereas Merged PiN-Schottky (MPS) diodes are not. A schematic explaining the thermal runaway mechanism is visible in figure 1: if we consider a simple cooling system (the plain line), it divides the Power/Temperature domain in two regions: in region A, above the plain line, the power dissipated is higher than what the cooling system can extract. Any system in region A will then tend to heat-up. On the oposite, a system in region B would tend to cool down, as the power it dissipates is lower that what the cooling system can extract. If we assume the imaginary device plotted in figure 1 (dashed line), we can see that there are two equilibrium points, where the power dissipated by the device is equal to what the cooling system can extract. However, the topmost equilibrium point is unstable: in particular, a small increase in the device junction temperature puts it in region A, where its temperature will increase indefinitely (that is, until the destruction of the device). (more explanations will be given in the full paper) In this paper, we performed I(V) measurements on a SiC JFET (SiCED 1200 V, 2.4x2.4 mm die) …