Study On Novel SiC MOSFETs For Improved Third Quadrant Performance

Due to the double advantages brought by the material and device structure,SiC MOSFETs have better switching characteristics than Sibased bipolar devices,and perform better in high frequency,high voltage and high temperature environment.With the improvement of substrate quality and device reliability,SiC MOSFETs have been successfully commercialized in recent years.Several fields such as electric vehicles,photovoltaic inverters and charging piles start to focus on SiC MOSFETs.The body diode of SiC MOSFETs is needed for freewheeling during switching process.But SiC diodes have high forward conduction voltage drop owing to the wide bandgap material characteristics,which increases the freewheeling loss of the device.In addition,the long-term recombination of electrons and holes during the conduction of the body diode will lead to bipolar degradation,and further deteriorating the reliability of SiC MOSFETs.One solution is to antiparallel a Schottky barrier diode（SBD）externally,but it will increase chip area and packaging costs,as well as introducing parasitic inductors and capacitors,resulting in weak high-frequency characteristics of the device.In order to overcome the above disadvantages,research on improving the third quadrant characteristics of SiC MOSFETs is carried out.By integrating low-loss diode and MOS-channel diode,the third quadrant turn-on voltage of SiC MOSFET is significantly reduced,and the bipolar degradation problem is suppressed.The main work of this thesis is as follows:（1）A novel SiC trench MOSFET with integrated low-loss diode（LLD-ATMOS）is proposed.The main feature of this structure is the introduction of an N-channel layer（N-channel）at the bottom of the trench.The conduction band energy in the fully-depleted N-channel region gradually decreases from the P-well side and reaches the minimum at the oxide interface,thus forming a low barrier.Since both sides of the N-channel region are adjacent to the N-CSL layer and the N+source region respectively,a low barrier path is provided to transport electrons from the drift region to the source region,which is equivalent to a low-loss diode（LLD）.When the device works in the third quadrant,the turn-on voltage of the LLD is only 0.75V,much lower than that of body diode（2.65V）.At the same time,the LLD has a better reverse recovery performance than the body diode owing to its unipolar conduction characteristics.The reverse recovery charge(Q_RR)of the LLD is reduced to one quarter of the body diode.In addition,the polysilicon gate splits into two parts,the top one is the true gate while the bottom one is the source-connected dummy gate.As a result,the gate-to-drain capacitance(C_gd)and gate-to-drain charge(Q_gd)are greatly decreased.By comparison,the turn-on and turn-off loss of LLD-ATMOS are reduced by 40%and 54%,respectively.The high-frequency figure of merit(R_on,sp×C_gd)of LLD-ATMOS is also tremendously decreased to one-fourteenth of the value of the traditional structure（C-ATMOS）.（2）A novel SiC trench MOSFET with integrated MOS-channel diode（MD-ATMOS）is proposed.In MD-ATMOS structure,an additional trench is etched in the left P-well region,named source trench.The polysilicon in the source trench is connected to source electrode,and forms a MOS structure with the oxide layer and P-base region.The right and left interfaces of P-base area are electron inversion channel and MOS-channel diode respectively.Owing to the surface electric field effect of MOS structure and the work function difference between N-type polysilicon and P-type silicon carbide,the energy band bends downward at the P-base surface.In addition,the extremely thin oxide layer of the source trench further increases the surface potential,thus reducing the barrier for electrons in the CSL layer to reach the source region.As a result,the turn-on voltage of MOS-channel diode is low to 1.4V,which effectively inactivates the body diode.On the other hand,the source and the gate trench are half-wrapped by the P-well region,which ensures that the electric field at the corner of the oxide layer is shielded and the device is reliable.The peak reverse recovery current and turn-on loss of the MD-ATMOS reduce66%and 47%without sacrificing other dynamic and static characteristics in comparison with C-ATMOS.The LLD-ATMOS and the MD-ATMOS proposed in this thesis improve the third quadrant characteristic,reverse recovery performance and switching loss performance compared with traditional structure.At the same time,the reliability of SiC trench MOSFETs is improved by suppressing the bipolar degradation effect of the body diode,thus providing two new device structures for the practical application of SiC power devices.