The Research Of The Power Mosfet Breakdown

Breakdown voltage is one of the key properties for power semiconductor device. In the different areas, the level of breakdown voltage for power semiconductor device varies. Power semiconductor is widely used in display driver, electric train, High Voltage Direct Current transmission and so on, in which, the breakdown voltage varies from100V to10000V. In order to acquire the breakdown voltages for different areas, breakdown has been studied since1960s. And the termination techniques were developed to improve the breakdown voltage.In this thesis, the study on floating field ring and field plate is carried on and a design methodology for termination is given. The easier topics are firstly studied and then harder ones.At first, the research on breakdown theory, the analyzing method and analytical solution are carried on. And some simulations are also made to verify the theory. The simulation results demonstrate that the analytical solutions for planar junction and cylindrical junction are very close to simulation results. The analytical solutions can be used in predicting the electric field distribution and potential distribution.Based on the research of basic junctions, floating field ring and field plate are studied, as well as the combination structure of floating field ring and field plate. Some simulation results are made, which can give some useful conclusions. The space between floating field rings, as well as the junction depth, has strong impact on breakdown voltage. The breakdown voltage for filed plate is sensitive to the length and the oxide thickness. The deeper the junction depth of field ring, the higher the breakdown voltage goes. With the enlargement of the field ring space, the breakdown voltage will increase and then decrease. The level of surface electric field is lowered by the field plate technique. Long field plate contributes to a higher breakdown voltage. And the optimization can also be carried on by adjusting the the thickness of the oxide underneath the field plate metal. The simulation demonstrates that field ring takes the advantage of improving the breakdown voltage and filed plate can effectively decrease the surface breakdown voltage.The combination of the filed ring and field plate has both the advantages of reducing the surface electric field and improving the breakdown voltage.Finally, in order to achieve the optimum breakdown voltage, the structure of the termination, the surface electric field distribution and the potential distribution are studied. The methods for the termination design are given. One is named after "equal peak electric field method" and the other one "equal potential method"The "equal potential method" is taken to design a500V multiple floating field ring termination. And the termination is optimized by introducing the field plate. The electric field distribution is optimized and the impact of the surface charge on the device reliability is improved. The simulation results show that the "equal potential method" are available for high voltage power semiconductor device termination design.A500V multi field rings termination is designed whose width is105um. And an optimization has been taken to make each field rings sustain the same voltage. Because of the reduction of the surface electric field, the breakdown point will be not located at the surface of each field rings.