Preparation And Characterization Of GaN Vertical Diodes

GaN-based materials have the advantages of large bandgap,strong breakdownekctrical field strength,high polarization coefficient,high electron mobility,and high electron saturation drift vetocity,and are the preferred materials for the preparation of new generation high-performance power electronic devices with important applicatioon prospects.In recent years,developed countries and regions such as the United States,Japan,and Emrope have all listed GaN-based power electronic devices im imajor strategic research plans and have made important progress.Until recently,although research on latoal structure devices such as AlGaN/GaN high electron mobility transistors(HEMT)is very popular,it still faces problems such as reliability and integration GaN vertical devices have recently attracted more attention.Compared with GaN lateral devices,vertical devices have several potential advantages:1)Higher current density without increasing chip size 2)Excellent reliability due to peak electric field away from device surface;3)Superior thermal dissipation performance.High-performance GaN vertical devices currently reported are mostly epitaxially grown on GaN bulk materials.However,this bulk material substrate is very expensive,and the size of the currently available epitaxial wafers is still limited to 2 inch.An inexpensive GaN epi-wafer is an important step towards the commercialization of vertical GaN devices.In this dissertation,GaN vertical diodes with avalanche breakdown capability are fabricated on sapphire substrate.Device simulation,device design,device processing optimization,device characterization,and so on are conducted.Besides,a bevel mesa structure is made to suppress edge-field crowding effect.The main results of this work are as follows:1.A GaN-based vertical diode with avalanche breakdown capability has been successfully fabricated.The breakdown voltage of the device is?3 60V,the forward conduction voltage drop is 3.4V,and the on-resistance is 2.848m?·cm2.The device has good performance,which is comparable to the best GaN diodes prepared on heteroepitaxial substrate reported so far.In UIS test,the avalanche breakdown voltage of the device is 380V and the peak avalanche current is?78 mA when a 47 mH inductor is used in the circuit.With a UIS circuit with smaller inductance,the device can withstand 100,000 times of avalanches without failure.As inductance in the UIS circuit increases,the total avalanche discharge energy that the device can withstand would decrease,and the device exhibits a certain degree of avalanche degradation.In order to study the basic mechanism of degradation,we conducted variable temperature?-? test and static repeated avalanche breakdown test,as well as Silvaco's numerical simulation and pulsed ?-? measurement.Based on these analyses,it is concluded that catastrophic thermal effect accumulated in device edge region due to local avalanche breakdown is the main cause of device degradation..2.To reduce strong edge electrical field crowding,GaN-based p-i-n diodes with beveled mesa termination ha-ve been successfully designed and fabricated.The ?-?characteristics and avalanche capability of the beveled mesa device have been tested and analyzed.The results show that the GaN-based diode with beveled mesa structure can well suppress edge electrical field crowding effect.Based on the same epitaxial structure,the breakdown voltage of the beveled mesa device increases to 480V with a turn-on voltage drop of 3.5V and an on-resistance of 9.52 m?.cm2.In order to better understand the effect of the beveled mesa to suppress strong accumulation of edge electrical field,we conducted detailed TCAD numerical simulation and avalanche luminescence test:The avalanche pattern of the beveled mesa is more uniform.Finally,in UIS test,the beveled mesa device shows excellent performance.For the UIS circuit with inductance of 1 mH,the avalanche voltage could reach 548 V with a peak avalanche current of 5.8 A.For the self-quenching phenomenon exhibited by the device,we proposed a charging and discharging rate model.