The Study On The Simulation Of 4H-SiC Floating Junction Power UMOSFET

The silicon carbide trench MOSFET(Si C UMOSFET) is an important structure of Si C power MOSFETs. Si C UMOSFET has a lower resistance at the same breakdown voltage compared with VDMOSFETs. The dielectric constant of the gate oxide is 2/5 that of Si C and the peak of electric field in the gate oxide is also intensified due to the electric field crowding effect at the sharp corner of the gate trench bottom, so the value of peak electric field of the gate oxide is very high, which results in dielectric breakdown and the decline of the device reliability. The drift region is unable to completely sustain the higher blocking voltage In this thesis, floating junction is introduced in the UMOSFET structure which not only shields the gate oxide at the bottom of the gate oxide from the high electric field during the blocking state and enhances the breakdown voltage(VBR).The effects of doping concentration(NA-FJ), length(L), position(d1) of FJ on VBR, electric field distribution, and specific on-resistance(Ron,sp) of Si C FJ UMOSFET are studied in the simulation by TCAD.(1) The gate oxide electric field can be effectively suppressed by higher NA-FJ and longer L is shown in the simulations, while too high values of these parameters will lead to a decrease of the breakdown voltage. Therefore, there is a trade-off between improvement of VBR and protection of the gate oxide. Also, the FJ should be located in the upper middle part of the drift layer in order to shield the high electric field of gate oxide. When the two electric field peaks of drift region are nearly equal, the breakdown voltage reachs a maximum value.(2) In addition, the variations of NA-FJ and d1 have little influence on Ron,sp but the Ron,sp value is improved by a decrease in L. The reason is that the width of current access varies small versus NA-FLI and d1, but the increase of L narrows the current access obviously.(3) According to the conclusions above, a group of suitable geometry parameters of FJ are obtained for 10μm/6×1015cm-3 of drift region. This not only shields the gate oxide at the bottom of the trench gate from the high electric field during the blocking state but also enhances the VBR from 611 V to 1541 V. The VBR and BFOM is increased by 150% and 440% respectively compared with the conventional UMOSFET.