Physical modeling of on-state losses in bipolar Si and SiC power devices

Sammanfattning: Power losses affect both the installation- and long-term cost of power electronic systems. The installation cost is related to the fatt that power losses in silicon power devices generate heat and make installation of heat sinks and water cooling necessary. If self-heating effects are strong, lotal overheating can eventually lead to device failure. To improve the design of power device systems, more accurate calculations of the Safe Operating Area (SOA) of power devices are desirable. Power semiconductor devices with lower losses are also needed. With the emerg ing SiC technology, much higher drift temperatures can be tolerated and much lower power losses can be achieved for the very high voltage range. In this thesis, on-state losses in bipolar Si and SiC power devices have been studied by comparing measurements to numerical simulations. Carrier distributions under high-leve1 injection were mesured utilizing the technique of Free Carrier Absorption (FCA). Measurements were performed for elevated temperatures under static equilibtium for Si power diodes and Insulated Gate Bipolar Transistors (IGBTs). Potential distributions in power diode structures were meas ured by contacting the samples with a tungsten probe tip. A set of physical models for accurate simulation of bipolar Si power devices is proposed; special attention was drawn to the modeling of minority carrier transport in emitters. Measurements of carrier distributions were canied out also for 4H-SiC power diode structures and the results were compared with simulation. Physical models for simulation of 4H- and 6H- SiC bipolar power devices are suggested. It was found that anisotropic material properties are important for the operation of bipolar 6H-SiC devices.Finally, various contributions to the heat generation term of a recently improved theory were evaluated under extreme, but realistic conditions. It was concluded that heat generation in bipo lar Si power devices, both stationary and transient, can be modeled accurately by only taking the Joule heat and the recombination heat terms into account.