The Simulation Study Of The 4H-SiC UMOSFET With Non-uniformly Doping Floating Junction

Silicon carbide(Si C) MOSFET is an excellent candidate for high power, high speed and high temperature applications. In recent years, the performance of Si C MOSFET has been improved with the developments of material growth and process technology. The Si C trench MOSFET(UMOSFET) has a low resistance compared with VDMOSFET due to the absence of the JFET region. But a critical problem in the UMOSFET is that the electric field will be crowded at the sharp corner of the gate trench bottom during the blocking state, which leads to the premature breakdown of gate dielectric and the reduction of device reliability. The Si C trench MOSFET with P+ floating island was proposed to adjust the electric field distribution of the drift layer and protect the gate oxide. Based on previous simulation results and analysis, there is a trade-off between the higher avalanche breakdown voltage and gate oxide protection. And the higher doping concentration of the floating island, the better protection of the gate oxide. But if the concentration of the floating island is too high, the device will be breakdown at the floating island in advance due to the intensity of the electric field crowding effect at abrupt depletion region border between FLI and the lower drift region. The best case is that avalanche breakdown almost simultaneously occurs at the positions where the two peak electric field locations. But in the actual process, the process cannot control so precise. The doping concentration of floating island need to be increased to make sure the protection of the gate oxide. However, the device will be breakdown at the floating island with the decrease of the breakdown voltage.In this paper, a novel 4H-Si C UMOSFET with non-uniform doping floating island is proposed. The feature of the non-uniform doping floating island is that the doping concentration of upper part is higher than the lower part. The peak electric field between floating island and drift region can be decreased by decreasing the doping concentration of the lower part. At the same time, high doping concentration for the upper part of the floating island is utilized, so that gate oxide layer at the sharp corner of the trench gate bottom cannot be broken down in advance. First, the forward and blocking characteristics of the dual-zone floating island UMOSFET are simulated. The simulation results indicate that the on-resistance of the device with non-uniformly doped floating island have nearly remained the same. In blocking characteristic, the gate oxide layer can be protected when the total charges of the floating island is above a certain value. And the breakdown voltage is closely related to the doping concentration of the floating island. The breakdown voltage is high when the doping concentration of the floating island changes gradually.On the basis of dual-zone floating junction analysis, the technological design of the Gaussian doping floating junction is provided. The breakdown voltage of the UMOSFET using a Gaussian doping profile in the floating island is 1644 V when the areal density of the floating island is the same, which is increased by 18.8% compared with the UMOSFET using uniformly doping profile in the floating island. More importantly, the breakdown occurs in the body and the dynamic performance of the device is not degraded. Finally, the switching and gate charge characteristics of the UMOSFETs both with uniform and Gaussian doping in floating island are simulated under the same charge of the floating island. The results show that the UMOSFET with Gaussian doping can improve the breakdown voltage of the device. At the same time, the dynamic performance is not degraded.