Study On Novel Reverse-conducting IGBT

As the essential core power device, IGBT is widely used in the green industry and products including new energy, high-speed rail, electric vehicle and smart grid. With the energy saving and the low-carbon economy prevailing around the world, IGBT will become a mainstream technology in the power semiconductor market. Since the invention of IGBT, it has been developing towards high power handling ability, high reliability, low power loss and low cost. In recent years, the rapid development of thin-wafer process technology and advanced chip design technology continued on pushing the performance improvement of the IGBT, which makes its key characteristics of forward conduction, turn off loss and short-circuit safe operation area reach an unprecedented level. In order to expand the development space of the IGBT, attempt of integrating IGBT and diode was made to develop the reverse-conducting IGBT products.Since IGBT has no reverse conducting capability, anti-parallel freewheeling diode(FWD) is needed in most of its application circuit. In the early age, IGBT and FWD were produced separately at first, and then co-packaged together to be IGBT-FWD pair. In order to cut down the cost and increase the power density, IGBT and FWD are integrated by process technology to develop RC-IGBT. After progressing for several years, the performance of the RC-IGBT improves rapidly, and the tendency of replacing the conventional IGBT-FWD pair becomes clear. However, there were still some drawbacks to be overcome, such as the voltage snapback problem during its forward conduction state, the non-uniform current distribution in the drift region and the poor reverse recovery characteristics. Those problems are barriers for the widespreading of RC-IGBT.The innovative content of this thesis are included in chapter 3 to chapter 5, which are briefly listed as follows:1. The reason of the voltage snapback in the conventional RC-IGBT is analyzed in detail, and then the condition to realize snapback-free characteristic is constructed. Based on the mechanism of its forward conduction state, the distributed resistances of the N-buffer above the P-emitter region with different layout design regions are found. Afterwards, the P-emitter length for RC-IGBT to be snapback-free is calculated. Finally, the calculation is verified by the simulation of Medici and the cause of the error is also investigated. The analytical calculation result is simple and accurate. It is suitable for Field-Stop RC-IGBT(FS-RC-IGBT) of various voltage classes and helpful for the RC-IGBT designer in anode layout deign.2. Based on the numerical simulation results, the phenomenon that the wide P-emitter region in the conventional RC-IGBT results in the non-uniform current distribution in the FWD and then causes the parasitic thyristor to be latch-up during its reverse-recovery process, is revealed for the first time. The lateral distribution variation of non-equilibrium hole with the time is analyzed from the RC-IGBT’s reverse conduction state to its reverse turn-off state, which reveals that the non-uniform distributed current is the root cause for the latch-up during the reverse recovery process. Finally, the influences of the lifetime, the length of the N+ short region and the impact of temperature on the immunity to the latch-up of the conventional RC-IGBT during its reverse recovery process are investigated.3. A series of novel RC-IGBT aiming to solve the voltage snapback problem are proposed. They not only solve the voltage snapback problem, but also have better tradeoff relationship between Von and Eoff(forward direction) as well as between Von and Qrr(reverse direction).Among them, two novel RC-IGBTs have distinguished forward and reverse conduction characteristics. Compared with the conventional RC-IGBT, their conduction voltages have at least 35% reduction. Besides, all of them have uniform current distribution, which enables their maximum turn-off current density two times bigger than the conventional one.