The Study On 4H-SiC Power VDMOSFET With Integrated SBD

Due to excellent material properties of 4H-SiC,it has been shown great potential for the application requirement of higher power because of smaller volume,lower power consumption and higher temperature in power devices.Among 4H-SiC power devices,because the 4H-SiC MOSFET has a simple driver and a fast switching speed as a switching device,it has been rapidly developed to commercial products of SiC MOSFETs with different voltage levels launched in recent years.Due to the poor reverse recovery characteristics and the reliability problems of the parasitic body diode of 4H-SiC MOSFET,an external SBD is usually anti-paralleled with 4H-SiC MOSFET as a freewheeling diode in practical applications to suppress the conduction of the body diode.The external freewheeling diode not only increase the system volume,but also the stray inductances is added.Therefore,many organizations have studied the integrated structure of VDMOSFET and SBD in recent years.However,there are still many shortcomings in the research on the integrated structure of VDMOSFET and SBD.First,the considerations for the area of the integrated SBD are needed to improve.Second,the effects of temperature on the MOSFET and SBD devices are not adequately considered.In addition,it is beneficial for the integrated structure to improve the reverse recovery characteristics and reduce the switching power consumption,but the conduction power consumption could be increased.There is also a lack of systematic discussion regarding the trade-off between the conduction power consumption and the switching power consumption,as well as the comparison of the different integrated structure of VDMOSFET and SBD.Therefore,these problems is systematically studied in this paper and the main work is as follows:The effect of integrated SBD on the characteristics of VDMOSFET devices is discussed theoretically.Then two-dimensional simulation studies on the typical split source structure(SS-VDMOSFET)and split gate structure(SG-VDMOSFET)are performed.According to the compromise between the device's conduction capability and freewheeling capability,an integrated SBD structure design method is proposed to ensure that the body diode would not conduct at high temperature.Two conclusions are obtained by this method:Firstly,the two integrated SBD structures do help to improve the reverse recovery characteristics of the device,whose the advantages are more obvious at high temperatures.Secondly,because of the increased specific on-resistance of the device,the current conduction capability is degraded.The current conduction capability of SS-VDMOSFET and SG-VDMOSFET is degraded by 24%and29.3%,respectively.Aiming at this phenomenon,a split trench source VDMOSFET structure(STS-VDMOSFET)is proposed by reducing the size of the P+ohmic contact region.Compared with the C-VDMOFET,the current conduction capability of STS-VDMOSFET is not degraded at all.And the switching power consumption is effectively reduced when the conduction power consumption is not increased.Therefore,the total power consumption of STS-VDMOSFET is less than that of C-VDMOSFET in the full frequency range,while the decreased amplitude is more significant as the frequency increases.At 300 K,the power consumption is decreased by from 9%at 10 kHz to 19%at 100 kHz.At 450 K,the power consumption is decreased by from 15%at 10 kHz to 31%at 100 kHz.The total power consumption of the C-VDMOSFET and the three integrated SBD structures under the same area and current load is also compared this paper.Because the current conduction capability of the SS-VDMOSFET and the SG-VDMOSFET is weak,the selected current load is smaller than that of the C-VDMOSFET and STS-VDMOSFET.It is found that the total power consumption of the three integrated SBD architectures in the circuit are reduced because of the improved reverse recovery characteristics,whose advantage is more significant at high temperature.On the other hand,there are also performance differences between the three integrated SBD structures.Because there is a trade-off between conduction and switching power consumption,the conduction power consumption of the STS-VDMOSFET is the smallest but the SG-VDMOSFET's is the largest,where the relationship in switching power consumption is just the opposite.Therefore,the STS-VDMOSFET is more suitable for working at low frequency and high current,while the SG-VDMOSFET is more suitable for working at high frequency and low current.In order to further increase the current conduction capability of the device,an integrated SBD structure of rectangle cells is also designed.The maximum operating current densities of the SS-VDMOSFET and the STS-VDMOSFET are increased from 114 A/cm~2 and 150 A/cm~2 to 134 A/cm~2 and 167 A/cm~2,respectively,while the SG-VDMOSFET of square cells has no advantage over strip cells.At the same time,the traditional analytical model is modified to obtain the analytical calculation result of the conduction power and switching power consumption of the rectangle cell.Comparing the power consumption of rectangle cells and strip cells under low current load,it is obtained that the integrated SBD structures of the strip cells has more advantages under small current load,and the square cells are more suitable for working at high current.