Design And Application Of 4H-SiC JBS Diodes

As a typical third generation semiconductor material,silicon carbide（SiC）shows attractive characteristics such as wide band gap,high critical breakdown electric field,high electron mobility and good thermal conductivity.As a result,SiC is an ideal material to manufacture power devices.Compared with traditional silicon（Si）based power devices,SiC devices can alleviate the contradiction between on-resistance and breakdown voltage,meet the requirements of power electronic systems for high power density,high switching frequency and low heat dissipation,and are developing rapidly.Since the first commercial SiC diode was produced in 2001,rapid progress has been made in SiC devices.Currently,4H-SiC junction barrier Schottky（JBS）diodes are the most widely used SiC devices which has the advantages of high switching speed,low on-resistance and low reverse recovery current,and can significantly improve the performance of power electronic systems.However,the breakdown voltage of 4H-SiC JBS diodes is easily affected by the interface charge in the terminal area.The terminal design of the diode is very important,and the degradation of electrical characteristics under high temperature,high voltage,high humidity and other stresses also needs to be solved.Therefore,this thesis studies the structure design,reliability and application scheme of 4H-SiC JBS diode through theoretical analysis and experimental exploration.The main research contents can be summarized as follows.1.The structure design and simulations of a 1200 V 4H-SiC JBS diode are carried out.Firstly,an epitaxial layer with a doping concentration of 8×10¹⁵ cm^-3 and a thickness of 10μm is selected through theoretical calculations.Then,the forward and reverse characteristics of the 4H-SiC JBS diode are studied through simulations to determine the optimal structure of the active region.Finally,two different terminal structures of the diode,the junction terminal extension（JTE）and the field limit rings（FLR）,are designed and investigated.1)The effects of lateral length and injection dose of JTE structure on the breakdown voltage are analyzed,and the influence of 4H-SiC/SiO₂ interface charge on the terminal electric field distribution is also discussed.When the charge density is greater than 1×10¹² cm^-2,the breakdown voltage of the device has an obvious downward trend.2)The uniform FLR structure and the non-uniform FLR structure are studied,respectively.The key parameters of FLR structure are the spacing and number of rings.Simulation results show that the terminal efficiency of uniform FLR is lower than expected.After considering the influence of interface charge,we determine to chose a 24-rings non-uniform FLR structure.When the interface charge density varies from 1×10¹² cm^-2 to 7×10¹² cm^-2,the breakdown voltage of non-uniform FLR remains stable.2.The 4H-SiC JBS diodes are fabricated and their electrical characteristics are studied in detail.The key process conditions such as high temperature ion implantation,carbon film sputtering and ohmic contact are optimized based on the simulation results,and the 4H-SiC JBS diode are successfully fabricated and tested.The temperature-dependent forward current-voltage（I-V）characteristics of the JBS diode with a FLR structure indicate that the forward transport mechanism changes from the thermal emission to bipolar conduction with increasing conduction current.The reverse I-V curve shows that the reverse current has a strong dependence on the temperature and voltage.At low bias voltage,the reverse current is dominated by the Schottky effect.When the voltage increases,the reverse leakage is dominated by the thermal field emission mechanism.3.Reliability experiments and failure analysis are performed on 4H-SiC JBS diodes.Four types of aging experiments are designed for 4H-SiC JBS diodes with the FLR structure to evaluate their long-term reliability under high temperature,high humidity and power loads.In the surge experiment,a single half-sine current pulse is applied to the diode with a pulse width of 10 ms,the result shows that the maximum surge current that the diode can withstand is 115A.In the high temperature and high humidity reverse bias experiment,the diode is placed in the high temperature and high humidity environment for 1000 hours under the premise of applying 600 V bias voltage to the diode.After aging,the breakdown voltage of a diode drops to about 900 V.The failure analysis is carried out by using scanning electron microscopy（SEM）and optical beam induced resistance change（OBIRCH）.In the intermittent operating life（IOL）aging test,the parameters of the diode remain stable after3000 power cycles.In the temperature cycle（TC）experiment,the devices are placed in the environment of 175℃and-55℃for 15 minutes,respectively.With the increasing cycle number,the forward voltage drop of several devices increase significantly,and reflective scanning acoustic microscopy results indicate that the failed devices are seriously stratified.4.The application of 4H-SiC JBS diode in the Boost power factor correction（PFC）circuit is studied.Compared with the traditional Si-based fast recovery diode（FRD）,4H-SiC JBS diode can effectively improve the performance of PFC circuit.The PFC prototypes are built by using the 4H-SiC JBS diode and Si FRD,respectively,and tested at a circuit switching frequency of 50 kHz.The test results show that,compared with the Si FRD,the prototype efficiency of the SiC JBS is improved from 97%to 98.13%when the output power is 1000 W.When the output power changes from 400 W to 1000 W,the operating temperature of Si FRD increases from 36.2℃to 96.6℃,while the temperature of SiC JBS increases only from 27.8℃to 47.8℃,indicating that the SiC JBS has obvious advantages in improving the performance of PFC circuit.