Silicon carbide (SiC)-diodes have been commercially available since 2001 and various SiC-switches have been launched recently. Parallelly, gallium nitride (GaN) is moving into power electronics and the first low-voltage devices are already on the market. Currently, it seems that GaN-transistors are ideal for high frequency ICs up to 1kV (maybe 2kV) and maximum a few 10A. SiC transistors are better suited for discrete devices or modules blocking 1kV and above and virtually no limit in the current but in that range they will face strong competition from the silicon insulated gate bipolar transistors (IGBTs). SiC and GaN Schottky-diodes would offer a similar performance, hence here it becomes apparent that material cost and quality will finally decide the commercial success of wide bandgap devices. Bulk GaN is still prohibitively expensive, whereas GaN on silicon would offer an unrivalled cost advantage. Devices made from the latter could be even cheaper than silicon devices. However, packaging is already a limiting factor for silicon devices even more so in exploiting the advantage of wide bandgap materials with respect to switching speed and high temperature operation. After all, reliability is a must for any device no matter which material it is made of.
The temperature-humidity-bias (THB) test is the standard for accelerated stress testing with respect to corrosion and other humidity driven degradation mechanisms. Usually, 1000 hrs tests at 85 degree Celsius and 85 percent relative humidity (85/85) are used to predict up to 25 years of operation. With regard to the respective standards asking for limited selfheating, the bias was commonly limited to 80 V. Nevertheless, recent THB tests on 1.7 kV IGBT modules have shown that higher bias is a more severe test condition. Failure analysis confirmed corrosion of the Al chip-metallization as well as Cuand Ag-dendrites as the relevant failure mechanisms. In order to determine the acceleration due to bias, 1.2 kV IGBTmodules were tested in THB at 65 percent and 90 percent of their nominal voltage V nom , respectively. A characteristic degradation consisting of three phases has been identified. The 2 nd phase seems to be determined by Al corrosion and a factor of about two has been estimated for the acceleration between the aforementioned test-voltages. Within the 3 rd phase, the devices stabilized probably due to localized self-heating. Thus, this degradation mechanism is kind of self-limiting, but the higher leakage increases the risk of thermal runaway, i.e. catastrophic breakdown especially when biased close to V nom .
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