The Study On The Reliability Of 4H-SiC SBD/JBS Diode

Wide bandgap semiconductor materials represented by silicon carbide?SiC?and gallium nitride?GaN?exhibit great advantages and broad application prospects in applications for high temperature,high voltage,high power,radiation,and so on.They are suitable for manufacturing power electronic devices.Compared with traditional Si-based and other common semiconductor materials and devices,they have many superior features,which will become more and more obvious in the global market in the next few years.4H-SiC JBS/SBD power diode is the most mature commercial SiC power device,which has already had a better basis of research and application.However,4H-SiC JBS/SBD power diodes still have some reliability problems to be solved in terms of the demands of the applications in high temperature,high voltage,high current,and fast switching speed.In this dissertation,the experimental and theoretical research on the reliability of 4H-SiC SBD/JBS diodes are carried out,and the main contents and innovations are as follows.?1?The basic theory and fabrication technology of 4H-SiC SBD/JBS power diodes are studied to provide a theoretical and experimental basis for reliability experiments.Firstly,the electrical and thermal parameters of 4H-SiC SBD/JBS diodes were systematically investigated,which can characterize the performance and quality of the devices in reliability experiments.Secondly,the physical models required for failure mechanism analysis are studied,including Schottky contact model,avalanche collision ionization model,incomplete ionization model,forbidden band width model,heat transportion model,and so on.Finally,4H-SiC SBD/JBS diodes with field-limited ring termination are fabricated and tested,which can be used in reliability experiments.?2?The degradation mechanism of 4H-SiC JBS diodes under high temperature storage stress was studied.The device had stable forward conduction characteristics under the 45 h275 ^oC high temperature storage stress in the air environment,and the Schottky barrier height,ideal factor and on-resistance remain unchanged approximately.However,as the storage time increases,the breakdown voltage of the device firstly decreases and then increases.In order to study the reason of device degradation,the high temperature storage experiment for4H-SiC MOS capacitors and SiC SBDs with the field plate structure,numerical simulation calculations,and avalanche breakdown point localization experiment were carried out.It is made clear that as the storage time increases,and the amount of negative effective charge at the SiO₂/4H-SiC interface in the terminal region firstly increases and then decreases,which degenerates the reverse breakdown characteristics of the device,and the breakdown voltage decreases firstly and then increases.Based on the experimental results,the carrier transport mechanism is speculated.In the high temperature storage process,the change of negative effective charge density at the interface is due to the combined effect from thermal electron emission,interface state and near-interface trap at the SiO₂/4H-SiC interface.?3?The degradation mechanism of 4H-SiC JBS diodes under repeative dynamic avalanche stress was studied.The forward conduction characteristics of the Schottky contact of the device are very stable under 10⁶ repeative dynamic avalanche pulses with amplitude of 1 A approximately,and the barrier height and ideal factor remain unchanged.However,as the number of repeative dynamic avalanche pulses increases,the breakdown voltage of the device firstly increases,then decreases,and finally increases.Through numerical simulations and avalanche breakdown point localization experiments,it is made clear that the number of the negative effective charges at the SiO₂/4H-SiC interface in the terminal region firstly increases and then decreases,causing the breakdown voltage to shift.The reason for the change of the effective charge density at the interface may be due to the trasportation mechanism of the combined effect of thermal electron emission,interface state and near-interface trap on electrons and holes at the SiO₂/4H-SiC interface.Finally,based on the device degradation mechanism,a 4H-SiC JBS diode with a novel field limiting ring termination structure has been designed and fabricated,which can effectively suppress the breakdown voltage shift induced by the change of effective charge density at the SiO₂/4H-SiC interface.?4?The failure mechanism of 4H-SiC JBS diodes under surge current stress was studied.It is found that the device was subjected to a combination of power,junction temperature,voltage drop and heat dissipation when the surge current signal was applied,leading to a voltage hysteresis in the transient I-V curve of the device.When the surge current increased to a critical value,the hysteresis,junction temperature,and voltage drop of the device would increase dramatically,resulting in the damage of the surface material and device failure.Through numerical simulations and the analysis of material in the device,it is found that the reason of the device failure is that a part of the Al and Ti in the active region has migrated,resulting in low barrier regions at some locations on the surface of the epitaxial layer,and thus reducing the breakdown voltage of the device.Finally,based on the device degradation mechanism,we have designed and fabricated 4H-SiC MPS diodes,which can effectively improve the tolerance of surge current for the device.?5?The failure mechanism of 4H-SiC JBS diodes under high dv/dt stress was studied.The device has stable forward conduction characteristics under high dv/dt stress of 1283V/ns,and the Schottky barrier height,ideal factor and on-resistance remain unchanged.However,after high dv/dt stress,the reverse leakage current of a part of tested devices was increased rapidly at high blocking voltages,which didn't originate from avalanche multiplication.Through numerical simulations,device testing and material observation experiments,it is speculated that the cause of the degradation of the reverse blocking characteristics may result from the depletion layer which expanded in a short time under high dv/dt stress,and thus the carriers in the depletion layer were drawn out rapidly.This physical mechanism caused an instantaneous high electric field at some locations,resulting in a slight damage in the device,which increased the reverse leakage current.