Research On 4H-SiC MPS Diode Of Snapback Effect Suppression Method

With the transition of semiconductor materials from silicon(Si)to third-generation wide bandgap semiconductor silicon carbide(SiC),devices have the characteristics of low forward conduction voltage drop,high reverse breakdown voltage,and fast switching speed,which are the main research directions of current power diodesThe merged PiN Schottky diode(MPS)is a bipolar power device based on the structure and characteristics of both the Schottky diode(SBD)and PiN diode.During the forward conduction of the MPS diode,the operating mode of the device switches from unipolar to bipolar,which causes an abrupt change and generates the snapback effect,resulting in circuit oscillation.Additionally,a high turning voltage of the snapback effect can lead to significant power dissipation,which can cause device failure.Therefore,it is particularly important to suppress the snapback effect by adjusting the device parameters and optimizing the structure.This thesis focuses on studying the methods to suppress the snapback effect in 4H-SiC MPS diodes.The main contents are as follows:Firstly,theoretical analysis of the device structure,working principle,and snapback effect mechanism is conducted to provide a basis for model establishment and parameter design.Then,process parameters are designed and the device structure is optimized,including P~+region doping concentration,drift region doping concentration,P~+junction depth,and drift region size.A double-layer P~+region MPS diode(DMPS)is designed to suppress the snapback effect.Finally,the effects of each parameter on the snapback effect and breakdown voltage characteristics are investigated through simulating the forward characteristics and breakdown characteristics of the MPS diode,combined with the internal carrier concentration distribution and electric field distribution under reverse bias.Through theoretical analysis and numerical simulation,the following results have been obtained:(1)Increasing the doping concentration of the P~+region can increase the injection concentration of holes,but the transition voltage changes slightly and the snapback effect is not significantly affected.However,if the doping concentration of the P~+region is too low,it may cause punch-through breakdown.Increasing the doping concentration in the drift region increases the forward bias at which the P~+region turns on,exacerbating the occurrence of snapback effect.Increasing the doping concentration in the drift region leads to a worse shielding effect of the space charge region,resulting in a decrease in the breakdown voltage of the device(2)Increasing the P~+junction depth can enhance the hole concentration in the drift region,effectively suppressing the snapback effect and increasing the forward current in bipolar mode.At the same time,increasing the P~+junction depth strengthens the shielding effect of the space charge region,improving the device’s breakdown voltage capability.Decreasing the drift region size can accelerate the device’s transition to bipolar mode,and the effect of suppressing snapback is significant.Increasing the drift region size can enhance the device’s breakdown voltage.(3)The buffer layer structure beneath the P~+region in the DMPS diode increases the effective junction depth,thereby suppressing the snapback effect.Increasing the width of the P~+buffer layer reduces the turning voltage,and the optimal forward characteristic is achieved when the buffer layer width is equal to the P~+region width.The breakdown voltage increases with increasing width of the P~+buffer layer.Increasing the thickness of the P~+buffer layer results in a more pronounced suppression of the snapback effect.After effective shielding of the Schottky contact by the P~+junction depth,the thickness of the P~+buffer layer has a small effect on the breakdown voltage.Increasing the width of the Schottky contact improves the forward characteristics under unipolar mode operation,but exacerbates the snapback effect.The breakdown voltage of the device decreases as the width of the Schottky contact increases.In summary,this thesis conducts simulation analysis on various parameters of 4H-SiC MPS and DMPS diodes,and investigates their effects on the snapback effect based on the results obtained.By optimizing the process parameters and improving the device structure,the snapback effect is suppressed,which has reference value for improving the stability and reliability of MPS diodes.