Design And Optimization Of Voltage-sustaining Drift Layer For GaN Vertical Power Device

Semiconductor power devices are widely used in various fields such as consumer electronics,data center,industrial control and so on,which assume the important functions of variable voltage,variable frequency,rectifier,and power amplification in power electronic system.However,the performance of the traditional silicon(Si)power electronic device is limited by the inherent characteristics of its materials,and it is difficult to meet the requirement of modern society for power devices.As a representative of the third generation of wide bandgap semiconductor materials,gallium nitride(GaN)materials have the advantages of wide band gap,large critical breakdown field strength and high saturation electron rate,becoming a powerful support to break through the bottleneck of the development of traditional power electronic devices.GaN power devices mainly include lateral structure and vertical structure.Compared with lateral structure,vertical GaN power devices have the characteristics of high wafer utilization,no current collapse effect and good thermal management,which have obvious advantages in the new generation of high voltage,high speed and high power application scenarios.And how to further improve the inherent contradictory relationship between breakdown voltage VB and on-state resistance Ron has become one of the research priorities for vertical GaN power devices.Besides,the uneven triangular electric field distribution inside the voltage-sustaining drift layer of conventional power devices also severely limits the further improvement of vertical GaN power device performance.To address these issues,this thesis proposed novel structural designs and analysis of the operating mechanism of the voltage-sustaining drift layer based on GaN vertical Schottky diodes(SBDs)and trench MOSFETs(T-MOSFETs),the main content is as follows:1.A vertical GaN floating island power device is proposed.Firstly,the forward and reverse analytical model is established,and the regulation process and internal mechanism of floating island structure on the physical quantity of the device such as the built electric field and carrier transport are systematically analyzed.At the same time,the structure modeling and numerical simulation analysis of the device are carried out,and the influence law and mechanism of the core design parameters of floating junction structure on the key performance of the device are revealed.Finally,the system optimization strategy of floating junction GaN power device is obtained.2.For the problem that the monolayer floating island structure can hardly meet the requirement by high-voltage(HV)GaN-based power devices,the design idea of HV type floating island structure is proposed.By inserting a spatially distributed p-type multi-layer floating island structure inside the drift layer and using its charge coupling effect,the influence mechanism of the relevant parameters on the reverse characteristics of the device and its optimal design are systematically discussed and analyzed,while the forward analytical model is established to explore the trade-off between reverse breakdown voltage and forward resistance of the device.3.A vertical superjunction device is proposed,using periodic N/P type alternating junction-type voltage-sustaining layer instead of the conventional resistance-type voltagesustaining layer,and using 2D electric charge field technology to achieve the optimization of the surface electric field to bulk electric field.A model of the field potential distribution of the superjunction structure is established and the intrinsic regulation mechanism of the superjunction structure and its influence on the device characteristics are explained based on GaN vertical structure T-MOSFET devices.The step-doping superjunction structure is also proposed,and the device mechanism analysis and optimization design are carried out by establishing a charge superposition physical model.The results show that compared with the superjunction structure,the step-doping superjunction structure can improve the breakdown voltage and process window effectively.