Novel GaN Vertical Power Device And Its Breakdown Mechanism

Driven by the ever-increasing demand of power application represented by automotive electronics,data center and portable terminal fast charging,Gallium Nitride(GaN)power electronic devices have attracted considerable attention from academia and industry.GaN vertical devices based on native substrates have higher crystal qualities(e.g.dislocation density lower than 1e6 cm^-2)and better heat dissipation.Theoretically,GaN vertical devices can break through the bottleneck of GaN HEMTs on breakdown voltage and realize kilovolt application.However,the concentration of Electric-field(E-field)at the junction edge will lead to the physical process of carrier avalanche multiplication and defect state charge storage.Focusing on the development trend and the current technology bottleneck of GaN vertical devices,this thesis focuses on the breakdown mechanism and E-field optimization.Based on the study of the leakage mechanism of conventional GaN Schottky barrier diode(SBD)and current aperture vertical electron transistor(CAVET),the junction termination techonology is proposed to improve the uniformity of E-field distribution.The main research contents are as follows:(1)The inherent disadvantage of vertical GaN SBD is that the tunneling caused by E-field concentration at the edge of Schottky junction leads to the deterioration of rectification characteristics.It is proposed to explore the effect of junction termination techonology on optimizing the E-field distribution.(1)Junction termination extension SBD(JTE-SBD)uses the p-GaN layer to block the electron transport process at the Schottky edge,and optimizes the surface E-field distribution.The breakdown voltage of JTE-SBD can be increased from 333 V to 2733 V,and the on resistance is only increased by 25%.(2)Bevel edge termination SBD(Bevel-SBD)uses etching process to promote the depletion of the drift region.Bevel edge termination with large angle and deep etching depth can transfer the breakdown position to the termination.The maximum termination efficiency of Bevel-SBD is 93.8%,and the on resistance is increased from 1.8 m?*cm~2to 3.0 m?*cm~2.JTE-SBD needs selective area doping process,and the conduction loss is relatively small.The process of Bevel-SBD is relatively simple,but it needs to etch a large area of drift region to significantly improve the blocking capacity.Taking the realization of 2.5 k V withstand voltage as an example,the increasment of on resistance of JTE-SBD is 27.8%lower than Bevel-SBD.The BFOM value is increased from 60.5MW/cm~2 of conventional SBD to 3.3 GW/cm~2 of optimized junction termination structure,which is conducive to promoting the application of vertical GaN SBD in AC/DC rectifier,DC/AC inverter and DC/DC conversion module of lidar and data center.(2)Aiming at the technical bottleneck of low breakdown voltage caused by E-field concentration at current barrier layer(CBL)of GaN CAVET,P-GaN Island-CAVET(PI-CAVET)is proposed to improve the uniformity of E-field distribution.Based on the study of the internal relationship between PI structure,E-field distribution and breakdown voltage,as well as the expansion of device depletion region,the breakdown voltage of PI-CAVET increases by 83%,and the BFOM value increases from 0.87 GW/cm~2to 2.63GW/cm~2.The improvement of the uniformity of E-field distribution is conducive to the improvement of the breakdown voltage.What's more,it inhibits the reliability degradation of the GaN power device represented by threshold voltage instability and current collapse orgintated from the trap related charge storage effect under high E-field.