Research On Structure And Reliability Of Split Gate Trench MOSFET With Low Gate Charge

Split gate trench MOSFET(SGT MOSFET)is the core of power management in motor drive and battery management module.The switching speed and power loss of the device will directly determine the working frequency and energy conversion efficiency of the system.Taking this as the starting point,this thesis studies the new structure and reliability of SGT MOSFET with low gate charge.Relevant problems are studied and discussed in detail through theoretical analysis and simulation.The main contents are as follows:Firstly,The split gate structure of SGT MOSFET extending to the drift region is connected to the source potential,which can not only be used as field plate(FP)to assist in depleting carriers in drift region and increasing doping concentration of drift region,but also shield the gate-drain parasitic capacitance,and finally reduce the dynamic and static power loss of the device.Therefore,the structural parameters of SGT MOSFET are optimized by simulation tool,and the effects of epitaxial layer concentration,body region concentration,trench size and oxide layer thickness on electrical characteristics of the device are deeply analyzed.Finally,the cell structure of 100V SGT MOSFET meeting the design requirements is obtained.Its breakdown voltage is 105.5 V,specific on resistance is 34.1 m?·mm~2,threshold voltage is 2.9 V,and gate charge is 6.55 n C/mm~2.The terminal region is designed with double trenches,and size of the outermost trench is larger than that of the cell region.The simulated breakdown voltage reaches 105.6 V,and the breakdown occurs at the bottom of cell region trench to avoid excessive concentrated current in terminal region.The restrictive relationship between breakdown voltage and specific on resistance of SGT MOSFET is difficult to reduce the conduction power loss of device on premise of the same breakdown voltage.The SFP(stepped field plate)structure is introduced to further optimize electric field distribution in drift region,enhance the auxiliary depletion to reduce specific on resistance and achieve lower conduction power loss.The breakdown voltage obtained by simulation is 110.5 V and specific on resistance is reduced to29.1m?·mm~2.However,the SFP structure enhances capacitive coupling between the gate and split gate,which has an adverse impact on switching process of device.Based on analysis of the gate and split gate structure of SFP-SGT MOSFET,the new HSFP-SGT MOSFET and NSP-SGT MOSFET proposed in this thesis have better gate charge characteristics.They remove the middle part of the split gate polysilicon and gate polysilicon to form hollow structures,fundamentally reducing parasitic capacitance between the gate and split gate and then improving gate charge and switching characteristics of device.The above conclusions are verified by TCAD simulation.Compared with SFP-SGT MOSFET,the gate charge Q_G of HSFP-SGT MOSFET and NSP-SGT MOSFET is reduced from7.81 n C/mm~2 to 6.43 n C/mm~2 and 6.11 n C/mm~2 respectively.Therefore,in switching circuit simulation under resistive load,the total power loss is reduced by 9.1%and 11.6%respectively at 1MHz frequency.The proposed HSFP-SGT MOSFET introduces a new electric field peak in drift region.The change of electric field distribution brings new reliability problems,that is,the distribution of interface trap charges induced by hot carrier effect have changed.In simulation,the holes are injected into silicon dioxide-silicon interface of the trench under electric field to form interface trap charges.Qualitative simulation is conducted to analyze the drift of breakdown voltage and specific on resistance of HSFP-SGT MOSFET under stress conditions,as well as the effects of doping concentration of drift region and structural parameters of HSFP on breakdown stability.After balancing the relationship between electrical characteristics and reliability of HSFP-SGT MOSFET,finally the optimization reference is given.