Novel Structures Of GaN-Based High Voltage Power MOSFET REBULF

The power switch transistors are core devices of the power electronics system.Gallium Nitride(GaN)has great application potential in power electronic systems due to its unique properties such as wide bandgap,high critical electric field,and high saturated electron velocity.Therefore,good performance GaN power switching devices with high threshold voltage,low on-resistance,low reverse leakage current,and high breakdown voltage are promising for future power applications.To deal with problems(e.g.low threshold voltage,non-uniform electric field distribution,poor reverse blocking capability,etc.)of conventional AlGaN/GaN HFETs,a novel lateral AlGaN/GaN BJB-HFET with buried-junction-barrier and a novel vertical power DH-VMOS with GaN/AlGaN/GaN double heterojunction were proposed in this work.The main research contents are as follows:(1)A novel high breakdown voltage E-mode AlGaN/GaN HFET with a locally buried P-GaN layer in the unintentionally doped GaN channel layer is proposed and it has following characteristics:1.?The buried-junction-barrier(BJB)structure parallel to the 2-DEG conduction channel is introduced by a P-GaN layer.On the one hand,in "OFF-state",a reverse-biased barrier for electrons is formed along the channel in GaN buffer,which effectively reduces the buffer leakage current of the device.On the other hand,the depletion region of the PN junction extends toward the drain as the reverse bias voltage increases.Therefore,the BJB structure redistributes the channel electric field and reduces buck field(REBULF).Especially,the maximum electric-field locates at drain-side edge of the gate is significantly reduced.Simulation results show that,compared with the structure without BJB,the maximum electric field peak of BJB-HFET with a gate-drain distance of 5?m is reduced by 60%,and the reverse leakage current is reduced by about 5 orders of magnitude(VDS=383 V),and the breakdown voltage is improved about 211%.2?As the BJB structure may perform as a back-barrier to lift-up the conduction band of the AlGaN/GaN hetero-junction,the device also features a higher threshold-voltage(3.4 V).(2)A novel enhancement-mode GaN vertical power MOS transistor with GaN/AlGaN/GaN double heterojunction(DH-VMOS)was proposed.This DH-VMOS has three characteristics:1?A lateral GaN-top/AlGaN/GaN double heterojunction is designed on the top vertical GaN drift region.The polarization effect of the GaN-top/AlGaN hetero-junction can be engineered by tailoring the GaN-top layer thickness,which enables flexibly modulation the threshold voltage(VTH)of the DH-VMOS.Simulation results show that the VTH of the DH-VMOS can be shifted from +2.9 to +4 V by increasing the GaN-top layer thickness from 5 to 40 nm.2?The 2-DEG at the lower AlGaN/GaN hetero-interface performs as part of the conduction channel of the device which is beneficial for device on-resistance reduction.3?The P-GaN is introduced in the vertical GaN drift region to optimize the electric field to achieve a high blocking voltage.This work also focuses on the impact of the key parameters of P-GaN body(e.g.physical dimensions,doping concentration)on devices characteristics,which is important for material growth and device design of vertical GaN power transistors.