Investigation On High-voltage,High-efficiency And High-speed Vertical GaN Power Diodes

With the development of technology level and transformation of economic structure,electric vehicles,data centers,high-speed railways,communication base stations and other emerging fields are developing rapidly and playing an increasingly important role,which requires critical requirements on energy demand and energy structure.Larger capacity,higher efficiency,and cleaner energy technologies are inseparable from the support of power electronics technology and its core power devices.After decades of technological progress in materials and structures,the performance of power devices has been continuously improving.The wide bandgap semiconductor gallium nitride(GaN)can achieve high breakdown voltage,low ON-resistance,low switching loss and other excellent performance of devices due to its excellent material properties,breaking through the theoretical performance limit of traditional silicon(Si)devices.Compared with lateral GaN devices based on foreign substrates,vertical GaN devices based on intrinsic substrates(i.e.,GaN-on-GaN devices)are capable to utilize the advantages of GaN material to a great degree and are expected to achieve higher breakdown voltage,higher current capability and more superior dynamic performance.Therefore,vertical GaN devices have received widespread attention and become an important direction of power devices.However,as an emerging technology,research on vertical GaN-on-GaN devices is still in its infancy,and there are still many challenges in materials,structures,processes,and theories.This thesis investigates the vertical GaN-on-GaN power diodes based on theoretical research and experimental exploration.4 types of high-performance GaN-on-GaN Schottky diodes(SBDs)and Pi N diode(PND)are fabricated through structure design and process development with key mechanism investigated.The main content and innovations are as follows:(1)Realization of high-voltage,low-resistance,low-turn-on-voltage vertical GaN unipolar diodes by novel edge termination and interface engineering.In terms of termination,novel termination structure suitable for GaN is proposed.The blocking leakage current can be reduced by over 4 orders with nitrogen-plasma treated termination(NT).The breakdown voltage can be increased by nearly 3 times with developed fluorine-implanted termination(FIT).The mechanisms of the terminations are verified by physical,electrical and simulation methods.In terms of active region,near ideal Schottky interface is realized with optimized fabrication process,enabling both conduction and blocking performance close to theory.The Schottky interface characteristics is accurately extracted by using multiple models.Furthermore,tunnelling-diode(TD)is realized by interface engineering utilizing the unique tunnelling-enhancement layer in GaN,enabling simultaneously realization of high breakdown voltage and low turn-ON voltage.Based on the above-mentioned techniques as well as design and development of complete process flow,3 types of GaN-on-GaN unipolar diodes are developed:(1)NT-SBD with breakdown voltage of 995 V and specific ON-resistance of1.20 m?·cm~2;(2)FIT-SBD with breakdown voltage of 800 V,specific ON-resistance of1.08 m?·cm~2 and low turn-ON voltage of 0.55 V(defined at 0.1 A/cm~2);(3)FIT-TD with breakdown voltage of 1020 V and low turn-ON voltage of 0.43 V(defined at 0.1 A/cm~2),which is favourable for low conduction loss.(2)Fabrication of high-performance vertical GaN PND and reveal of evidence of conductivity modulation in dynamic level and its impact,as well as simultaneous realization of conductivity modulation in conduction and zero reverse recovery during turning-OFF.Based on the development and optimization of key process such as nitrogen-implanted termination(NIT)and high-quality metal/semiconductor contact,high-performance vertical GaN PND is fabricated with breakdown voltage of over 1.8 k V and conduction current of over10 k A/cm~2.The bipolar carrier transport in the vertical GaN PND is studied.The hole injection in conduction mode is investigated based on multi-scale characterization.The experimental evidence of conductivity modulation in the vertical GaN PND in dynamic level is provided.Through the unique photon-electron coupling effect in direct-bandgap GaN devices,the favourable conductivity modulation at forward bias and zero reverse recovery during turning-OFF can be simultaneously realized,which breaks through the limit of conduction and switching performance in traditional bipolar devices due to minority carrier injection.The impact of conductivity modulation on dynamic ON-resistance,surge current and other characteristics is also investigated.(3)Characterization of dynamic performance in high-speed board-level tests and verification of current-collapse-free performance in vertical GaN devices.The dynamic performance of the vertical GaN diodes is investigated.The transient conduction performance,reverse recovery performance and dynamic ON-resistance performance are characterized by using custom-designed test board.The transient conduction performance of the vertical GaN devices is investigated.Also,the surge current capability of the vertical GaN devices is studied.The current-collapse-free performance has been verified in the vertical GaN devices in fast board-level test,outperforming the commercial lateral GaN devices where dynamic ON-resistance degradation is still regarded as a challenge.By a series of systematic board-level tests,the superior dynamic performance and reliability are verified in vertical GaN-on-GaN devices,showing great potential in high-frequency and high-power power electronic applications.Based on 4 types of developed devices(NT-SBD,FIT-SBD,FIT-TD,NIT-PND),this thesis studies key technology of high-voltage,high-efficiency and high-speed vertical GaN power diodes.In terms of blocking performance,novel terminations are designed and realized while the active region is optimized by interface engineering,and the mechanisms are revealed.In terms of conduction performance,the unipolar devices utilize tunnelling-enhancement layer to reduce conduction loss while the bipolar devices facilitate high conduction capability and low conduction loss by photon-electron coupling.In terms of dynamic performance,the bipolar devices can exhibit fast reverse recovery performance which is similar with unipolar devices thanks to the ultrashort minority carrier lifetime,and the vertical GaN devices can also exhibit current-collapse-free performance due to its vertical structure and homoepitaxy layer.This work shows the great potential for vertical GaN-on-GaN devices.This work and subsequent research are of importance for vertical GaN-on-GaN devices to achieve high-voltage,high-efficiency and high-speed performance with better understanding,as well as promote their applications.