Design,Process And Modeling Investigation On Silicon Carbide MPS Diodes

Electricity is the main form of energy consumption nowadays,and its proportion is still increasing year by year.Therefore,power electronic technology for processing and transforming electric energy becomes more and more important.Semiconductor power devices are the core components of power electronics.In recent years,the new generation power devices based on silicon carbide(SiC)materials have sprung up.With their high breakdown voltage,low onresistance,and fast switching speed,they have gradually gained the favor of academia and industry.During the improvement of SiC devices,high-efficiency device design methods,stable and low-cost fabrication process,and studies of device behaviors and reliability under abnormal operating conditions all require careful researches.The silicon carbide diode is an excellent platform to study these issues.The most popular power diode types include Junction Barrier Schottky(JBS)diodes and Merged PiN Schottky(MPS)diodes.They achieve good balance between forward conduction and reverse blocking characteristics,and surge and avalanche reliability.Many researchers have done intensive researches on the relationship between MPS/JBS diode cell designs and device performance,and several manufacturers have already developed mature commercial products.However,there still remain some problems to be solved: the stable and reliable P-region Ohmic contact process in the SiC MPS diode,the design and extraction of the chip epitaxial layer parameters,the characterization of the electrical and thermal behavior of the device during the high-power electrothermal coupling process such as surge,and the acquisition of junction temperature information,etc.Focusing on these problems,this thesis designs and fabricates a variety of SiC MPS/JBS diodes,and carries out systemic characterization measurement and modeling analysis including:(1)structural parameter design and process development;(2)measurements of static & dynamic characteristics,and tests of surge reliability of the fabricated SiC MPS/JBS diodes;(3)establishment of the extraction method of the epitaxial parameters of vertical type power devices with field limited ring terminations;(4)establishment of the electro-thermal coupling model for surge process junction temperature calculation.This thesis has the following innovations:(1)Through the design,process,and testing of SiC MPS diodes with two cell arrangements and multiple sets of dimensional parameters,this thesis fully understands and masters the R&D technology of SiC MPS diodes.This thesis also reveals the connection between device cell design,static characteristics and surge reliability from both simulation and actual levels.According to the difference of current and temperature,this thesis simplifies the electrical behavior of SiC MPS diodes during the surge into three modes,and analyzes the formation and transformation mechanism of each mode in detail to deepen the understanding of their surge characteristics.This thesis also developed a set of SiC MPS diode process flow based on implanted P+ regions,which is compatible with the process flow of SiC JBS diode to the maximum extent.The devices fabricated according to this process flow have stable electrical characteristics,and exhibit high reliability comparable to commercial devices under extreme conditions such as surge current impact.(2)This thesis improves the conventional design and extraction methods of epitaxial layer parameters of power devices.By introducing auxiliary functions and combining numerical methods,this thesis proposes an algorithm which can directly processes the avalanche breakdown criterion in the form of double integral and calculates the avalanche breakdown voltage without introducing the assumption that the impact ionization coefficients of electrons and holes are equal.Based on this algorithm,this thesis gives a fitting formula suitable for 4H-SiC material to determine the best epitaxial parameters according to the design goal of the avalance breakdown voltage,which greatly facilitates the design of epitaxial layers.This thesis also improves the conventional C-V method of extracting the epitaxial layer parameters.The influences of the field limited rings(FLRs)on the depletion region shape and C-V characteristics of the device are considered.The optimized algorithm for extraction of epitaxial layer parameters proposed in this thesis gives out more accurate epitaxial doping concentration and thickness than the conventional C-V method,which is benefit for reverse engineering analysis of fabricated devices.(3)Based on the conventional RC thermal circuit model,this thesis proposes a distributed heat source electrothermal coupling junction temperature calculation model suitable for surge process.This model simulates the distributed heat source by changing the topology of the thermal branch.By changing the parameters of the electrical branch and the thermal branch successively,the decoupling of the electrical and thermal processes is achieved.It accurately and quickly predicts electrical behavior and internal junction temperature changes of the device during the surge from the static forward electrical characteristics and thermal impedance test results without actual surge testing.This model takes into account the fact that the heat sources are scattered throughout the chip instead of being concentrated at the main junction.It also considers the changes of the thermal resistance and heat capacitance of each material layer with temperature.It is closer to the actual situation compared with the conventional method,and thus has higher accuracy.The device design,process technology,modeling analysis and other research methods proposed in this thesis provide a complete methodology for device researchers.These means can speed up the design and analysis process of the devices,and deepen the understanding of the working mechanism of the device.It is foreseeable that this thesis and its follow-up researches will provide more and more research methods and mechanism models for power devices,which will greatly help the development of power devices.