Research On Structure And Performance Of Star-stepping Buffer Gate Metal Semiconductor Field Effect Transistor

As a third-generation semiconductor material,silicon carbide(SiC)is a characteristic semiconductor material which presents many unique features such as high critical breakdown electrical field,high electron saturation velocity,high thermal conductivity,and wide band gap.Therefore,SiC is the first choice of material for high temperature,high breakdown voltage,high frequency,high power,and anti-radiation devices.Due to the excellent characteristics of SiC materials and its increasingly mature manufacture processing,the metal semiconductor field effect transistor based on 4H-SiC(4H-SiC MESFET)has become the prior choice for a new generation of semiconductor devices,especially in high temperature,high electric field and strong radiation environment.However,there is a potential offset between the high saturation drain current and high breakdown voltage in the 4H-SiC MESFET,which limits the improvement of the power characteristics of the device.Generally,increasing the breakdown voltage will cause the saturation drain current density to decrease,and vice versa.Therefore,increasing effectively the breakdown voltage and saturation drain current density to meet the demand for high voltage and high power density has always been a research hotspot in this field.Focusing on this problem,the structure of 4H-SiC MESFET is optimized,and a new stair-stepping buffer-gate 4H-SiC MESFET(4H-SiC SBG-MESFET)is proposed in this thesis for the first time to improve the breakdown and power characteristics of the device.The terminal technology of breakdown point transfer(BPT)is applied in 4H-SiC SBG-MESFET in order to scatter the electric field lines and transfer the breakdown point.The one-dimensional and two-dimensional electric field distribution of SBG-MESFET,and its two-dimensional electrostatic potential profile are simulated by using ISE-TCAD in order to reveals the breakdown mechanism inside the device.The simulation results show that,compared with BG-MESFET,the introduction of breakdown point transfer technology transfers the electric field peak from left corner of the drain of SBG-MESFET to the edge of the field plate in the Si₃N₄ region between the gate and the drain,and thus dispersing the electric field distribution,making its distribution more uniformly and improving the breakdown characteristics of the device.Compared with BG-MESFET,the breakdown voltage of SBG-MESFET is increased from 120V to 180V,improved by the rate of 50%,and the power density is increased from 9.35 W/mm to 13.2 W/mm.On this basis,the structure of the 4H-SiC SBG-MESFET is further optimized and a novel stair-stepping buffer-gate 4H-SiC MESFET with multiple recesses(SBG-MR4H-SiC MESFET)is proposed.In order to improve the power characteristics of the device as far as possible,SBG-MR MESFET introduces double grooves in the p-type buffer layer and further optimizes the relative position,lateral width and longitudinal depth of the two grooves.The introduction of the p-type buffer layer grooves increases the channel thickness and cross-sectional area,thereby further improving the drain current density of the device.At the same time,because there is an isolated small mesa with a width of 0.2?m between the two recesses in the p-type buffer layer,the electric field line distribution of the 4H-SiC SBG-MR MESFET is further dispersed,effectively avoiding the problem of breakdown voltage decrease due to the increase of channel thickness.Using ISE-TCAD,the saturation current density and breakdown voltage of SBG-MR MESFET and SBG MESFET are simulated and analyzed,hence the power density of the two structures is compared,and the electron flow path and surface electric field line distribution are further analyzed to determine the breakdown point location.The simulation results show that compared with the SBG structure,although the breakdown point of the SBG-MR structure still occurs at the edge of the field plate and the maximum electric field peak value is not reduced,the electric field line distribution is more uniform and reasonable,and its breakdown voltage is increased by nearly 10%.The saturation drain current density is 37.5%higher than that of the SBG structure,so that the maximum output power density of SBG-MR MESFET is increased to48.7W/mm,which is 55% higher than that of SBG-MESFET.