Research On Key Technologies Of High Voltage FSRD

The fast and soft recovery diode(FSRD)provides a freewheeling loop for the inductive load in the power converrsion circuit.With the development of high voltage IGBT,the performances of anti-parallel diodes,including the low loss,the fast and soft recovery characteristic,the robustness and the surge current capability,should be improved.At present,the high voltage FSRD is difficu lt to make a better compromise between high surge current and dynamic avalanche capabilities.In addition,there are several problem s such as the electric field shifting,the current filament evolution and inhibition,the junction termination failure to be explored urgently.Based on the above problems,the key technologies of high volltage FSRD are studied in this paper,and the main contents are as follows:Firstly,the static and dynamic characteristics of high voltage p+pn-nn+ diode are investigated.The key factors affecting the zero temperature coefficient point(ZTC),the surge current characteristic and the reverse recovery characteristic are analyzed.Moreover,the formation mechanisms of positive and negative differential resistances and Egawa-field after static avalanche,and the induced factors of three-degree dynamic avalanches are discussed.Secondly,the mechanisms of the peak electric field shiftin g at the junctions after avalanche in high voltage FSRD are analyzed by establishing the electric field gradient analytical models,and the inhibition mechanism of the positive differential resistance induced by the peak electric field shifting on the current filament is studied.It points out that the appropriate buf-fer doping concentration is important to improve the dynamic robustn ess of the device.Simultaneously,the influence of carrier lifetime distributions on the evolution of current filaments and the inh ibition of local low carrier concentration on current filaments are explored.Thirdly,a novel anode shorted diode(ASD)is proposed,and the operating mechanism of the device is analyzed.The conduction condition of the parasitic npn transistor at the anode side is derived,and the factors affecting the electron injection efficiency are given.The results show that the diode exhibits a low forward voltage drop at higfi on-state current density,improving the surge current capability.During reverse recovery,the parasitic transis.tors of both sides conduct,inj ecting electrons and holes into the pn-junction and nn-junction to inhibit the peak electric fields,respectively.This improves the dynamic avalanche capacity.Fourthly,the failure mechanism of the junction termination of high voltage FSRD is studied by electrothermal simulation.During reverse recovery,high electric field will punch through the termination surface,resulting in the positive feedback process maintaining among the current filament,high electric field strength,and impact ionization.This causes an increase in temperature at the local region,leading to the termination failure.Then,in order to solve the termination failure,and based on the above-mentioned failure mechanism,two new termination structures including the improved trench-field limiting ring and the corrugated p-type resistance region are proposed,which reduces the current filament at the termination surface and transfers the peak electric field into the body,avoiding the positive feedback process.Electrothermal simulation shows that the dynamic ruggedness of both terminations are improved.Finally,the process implementation scheme of 3.3 kV/100A FSRD is developed by the process research and simulation,and the experiments are carried out.The test results show that the forward voltage drop of 2.192 V at a rated current of 100 A,the reverse breakdown voltage of 3.5 kV,the reverse recovery time of 652 ns,and the softness factor of 0.67 are obtained,and it shows a fast and soft recovery characteristic.