Research On The Mechanism And Model Of Wide Bandgap Semiconductor Microwave Devices

With the increasing demand of science-technicalization and informatization in the area of national defense weapons and social production,the electronic-information industry has shown a rapid development trend in recent decades,especially considering the coming of 5G communications and equipment informatization,higher requirements are demanded on the performance of radio frequency power devices.The third-generation widegap semiconductor materials such as gallium nitride(GaN),silicon carbide(SiC)and Diamond are the basis of the rapid development of microwave power devices in terms of high-performance and miniaturization in recent years.Among them,GaN High Electron Mobility Transistor(HEMT)is the most representative.However,due to the complicated physical effects during the operation of GaN HEMT devices,further research on the working mechanism and characterization is needed.As a bridge between device fabrication process and circuit design,device model is very important for further research on the working mechanism of GaN HEMTs.At present,the existing empirical equivalent circuit model(compact model)generally has the defects such as many fitting parameters,difficulty in extracting parameters,unclear physical meaning and difficulty in associating device processes.With the emergence of new materials and structures,the empirical model is not convenient to embed and characterize physical effects,which is not conducive to the update models with the fabircation process upgrading.The physics-based compact model can solve the problems.In this dissertation,based on the semiconductor device theory,an accurate physics-based compact model with clear physical meaning is established which is suitable for circuit design.After research on the electro-thermal and electro-mechanical effects,the analytical thermo-electron coupling model and electro-mechanical coupling model are established for GaN-on-Diamond HEMT and flexible AlGaN/AlN/GaN HEMT devices,respectively.The main research content is as follows.(1)Fermi potential E_f and two-dimensional electron gas density n_s analytical model.With the disadvantage on the equation continuity and iterative computation,it is difficult to integrate existing physical models into the compact model for circuit design and application.Therefore,an analytical modeling method to accurately characterise Fermi potential and 2DEG(two-dimensional electron gas)density is proposed.Firstly,by studying the formation mechanism of 2DEG in triangle well,the analytical expressions of E_fand n_sabout gate bias are derived in each region,respectively.Then,the continuous,unified analytical model which is appropriate for the whole operation region is established by introducing region-transition function.The proposed model is verified by comparison with the numerical solution,showing excellent coincidence degree of more than 98%and good continuity,which is the foundation for the establishment of following large-signal physics-based compact model.(2)Research on large-signal compact model of GaN HEMTs based on surface potential.In order to overcome the defects of empirical equivalent circuit model including large amount of fitting parameters,difficulty in parameter extraction and lack of physical meaning,as well as the shortcomings of traditional surface potential model,such as the complex iterative calculation and the difficulty to embed implicit expression into circuit design,a physics-based large-signal compact model based on surface potential is proposed in this dissertation.Firstly,I-V and Q/C-V characteristic equations are derived,which including multiple physical effects characterization,such as the self heating effect,trapping effect,the carrier velocity saturation effect and breakdown effect.Combined with the principle of separate region scaling principle,a scalable physics-based large-signal compact model is established.The good continuity and symmetry of model are verified through the comparison and verification of domestic0.25?m X band GaN HEMTs with diffdrent gate width.Results show that the DC-IV,S parameters,large signal power,efficiency and harmonic output power of the device can be characterised accurately by the proposed model.The model meets the requirements of circuit design with output power and power added efficiency simulation accuracy more than 94%,which meets the circuit design requirements.Compared with Angelov empirical model and latest reported quasi-physical zone division model,the amount of parameters in our model is reduced by 47%and 24%,respectively.Furthermore,the proposed model is easy to the embedding of physical effects.(3)Research on thermal effect and compact model of GaN-on-Diamond HEMTs.Aiming at solving the problems of the inaccuracy of the junction temperature prediction caused by ignoring the interface thermal resistance or simplifying it by a fixed value in the reported thermal model,a three-dimensional thermal simulation model is established based on the finite element method,which is able to reflect device channel junction temperature accurately.The thermal effect of bonding layer is characterized by the physical analytical method with clear physical meaning.The extracted analytical thermal resistance model related to structure size,material parameters and dissipated power is embedded into the large signal model to build the large-signal thermo-electron coupling model finally.Compared with the infrared thermal imaging measurements,the accuracy of thermal model reaches 97%with 10%improvement.The accuracy of output power and efficiency of large-signal thermo-electron model is more than 95%.Through performance comparison before/after substrate transfer and junction temperature analyzing,it shows that the proposed model can be used to study relationship between fabrication process and thermal characteristics,as well as the mechanism after introducing diamond substrate and its influence on the large signal characteristics.(4)Research on the physical and analytical electro-mechanical coupling model of flexible GaN HEMTs.The reported strain models of flexible GaN HEMTs are mostly three-dimensional finite element models,which are difficult to be embedded in the circuit simulation software for the further study of power characteristics.Few analytical models just stay at the level of qualitative analysis of drain current,which are not allowed to explain the phenomenon of opposite strain characteristics under the same strain.Considering the above problems,a electro-mechanical coupling model for flexible GaN HEMTs is proposed in this dissertation.The strain effect is introduced into the derivation of 2DEG density equation,considering the effects of piezoelectric polarization,Schottky barrier height,band structure and surface state density.The output characteristics of GaN HEMTs can be predicted accurately under different strain conditions,and the phenomenon of opposite strain characteristics under the same strain is also explained,which makes the model more pratical.Through the fabricated flexible AlGaN/AlN/GaN HEMT on parylene substrate,the simulation accuracy of model on the threshold voltage,2DEG concentration,DC-IV and power output characteristics under different strain conditions is verified.The accuracy of output power and power added efficiency is more than 96%.The proposed model is beneficial to the deep understanding of GaN HEMTs strain characteristics and performance improvement through strain engineering.