The Study On The Design And Experiment Of Advanced 4H-SiC SBD/JBS

As typical one of the third generation of semiconductor materials,silicon carbide?SiC?becomes an ideal material for high voltage,high temperature,high power and radiation hardening power electronic device.Compared with traditional Si power device,4H-SiC power device has a better tradeoff between breakdown voltage and specific on-resistance.Meanwhile,it could improve the operating temperature and frequency,lower the power loss of a system,resulting in a smaller size,weight and cooling equipment application system could be applied in the field of electric cars,rail transportation,smart grid and space applications,etc.As the most common studies on 4H-SiC device,4H-SiC Schottky Barrier Diode?SBD?and Junction Barrier Schottky Didoe?JBS?have been studied by many groups and achieved some remarkable achievements.However,in some areas,such as termination optimization,reliable problem and new device structure design,still have many problems to be solved.These problems restrict the performance improvement of 4H-SiC Schottky device and large scale applications in power electronic system.In this work,both theoretical simulation and experimental studies are performed to improve the performance of 4H-SiC SBD/JBS.The main research contents and innovations are as follows:1)The physical and simulation models of 4H-SiC SBD/JBS are studied.Firstly,the ideal specific on-resistance and breakdown voltage model of 4H-SiC SBD/JBS are introduced.The relationship between BFOM and parameters of epitaxial layer is set up.According to the design goal,the suitable parameters of epitaxial layer are fixed.Secondly,reasonable device models are built,based on relevant suitable physics and material parameters,which include incomplete ionization model,avalanche generation model,Schottky contact model and traps and fixed charges model.2)High performance 4H-SiC JBS is studied.Based on the operational mechanism of field limiting rings?FLRs?termination structure,a Non-uniform FLRs termination structure for 4H-SiC JBS is designed.The effect of key parameters,such as?S,S1 and Nr on the breakdown characteristic is studied too.Subsequently,the fabrication process of FLRs is implemented based on the FLRs simulation results.Finally,the 1.2 kV,3.3 kV and 5 kV4H-SiC JBSs with Non-uniform FLRs structure are fabricated based on three different epitaxial layer parameters.It is shown that the breakdown voltages of different devices could reach to 1.65 k V,3.7 kV and 5.7 kV respectively.The termination efficiency is 92.7%,95%and 76%respectively.Meanwhile,the experimental results demonstrate that S₁ and N_r have a great influence of the performance of FLRs structure,which is agreement with simulation results.3)Research on New Trench Multiple floating limiting rings?TMFLRs?.Based on the avalanche breakdown mechanism,a TMFLRs structure,including trench structure and conventional FLRs structure,is introduced to improve the FLRs performance in ultrahigh voltage device without extra fabrication process.The new structure could relieve the peak electric field at the corner of the rings by increasing the junction depth to increase the breakdown voltage easily.The simulation results show that the trench depth and width of the TMFLRs structure are the key parameters for TMFLRS design.Suitable trench structure could relieve peak electric field in the bulk and interface simultaneously.Finally,the 4H-SiC JBS with both conventional FLRs and TMFLRs are fabricated at the same time on a lightly doped epitaxial layer(thickness=50?m,doping=1×10¹55 cm^-3).A maximum breakdown voltage of 6.7 k V is achieved with TMFLRs structure,yielding about 90%of the parallel plane breakdown voltage.Compared with the conventional FLRs structure,the new structure makes the breakdown voltage and termination efficiency increase by about 1kV and 14%respectively.4)Research on dynamic avalanche characteristic of 4H-SiC power diode and structure optimization.Through the repetitive avalanche current measurement of 1.2 kV 4H-SiC JBS,it could be found that the breakdown voltage of the fabricated device shifts with the increase of repetitive avalanche pulse number.With the help of physical structure analysis of the failed device and theoretical simulation,it is indicated that there is hole accumulation at the interface of SiO₂ and 4H-SiC between two rings during the avalanche current stress,resulting in the breakdown voltage shifts.In order to decrease the sensitiveness of the FLRs structure to the interface charge,the designed FLRs structure is optimized again.According to the simulation results,it could be proved that the sensitiveness of FLRs structure to the interface charge could be decreased when reducing the space between two rings and increasing the number of rings.5)Structure design and experiment research of 4H-SiC FJ_SBD/JBS.Firstly,one-dimension analytical models of specific on-resistance and breakdown voltage of FJ_SBD are built based on the operational mechanism of floating junction device.Compared to the simulation results,the validity of the analytical models is proved.And then the epitaxial layer parameter to be suitable for floating junction device is designed with according to analytical models and simulation results.Secondly,floating junction structure in the termination region and termination structure are analyzed.It is shown that the device reverse performance is affected by the the doping dose if the floating junction structure in the termination region is continuous.For discontinuous floating junction structure,its performance is sensitive to the width and space.Considering the compatibility of fabrication process,the JTE termination structure is designed for floating junction device which possesses discontinuous floating junction structure in the termination region.Finally,4H-SiC FJ_SBD/JBS are fabricated based on the design approach mentioned above.The measurements show that the maximum breakdown voltage of 3.5 kV and the specific on-resistance of 5.67 m??cm² are achieved for the fabricated FJ_JBS device.These values are reported first time,with corresponding to the Baliga's figure of merit of 8.64 GW/cm²,which is better than FJ_SBD and conventional device on same wafer.