Study Of Mechanism And Novel Structure Of High-Voltage Enhancement-Mode GaN HFET

AlGaN/GaN Heterojunction Field-effect Transistors?AlGa N/GaN HFETs?are considered to be one of the promising candidates for replacing silicon-based power devices in the future due to their superior material properties and great device performance.The two-dimensional electron gas?2DEG?of high sheet density and high electron mobility formed by the AlGaN/GaN heterojunction makes the conventional AlGaN/GaN HFET is intrinsically depletion-mode and also limits its application in power electronics.Additionally,the electric field crowding effect at the gate edge and leakage current of GaN buffer layer may lead to premature breakdown of AlGaN/GaN HFET in the off-state,resulting in that the average breakdown electric field of the device is far lower than the critical breakdown electric field of GaN material.In order to solve the above problems,two novle AlGaN/GaN HFET power devices are proposed in this thesis.Their mechanisms,static characteristics and key parameters are also investigated.1.For the conventional AlGaN/GaN HFET,the breakdown voltage?BV?is far lower than its theoretical limit and there is a trade-off between the threshold voltage and the on-state current.A symmetrically polarization-doped enhancement-mode AlGaN/GaN HFET?SPD-HFET?is proposed.It featurs back-to-back graded AlGaN barrier,which forms polarization gradient and then induces the three-dimensional electron gas?3DEG?and three-dimensional hole gas?3DHG?in the positive and negative graded AlGaN layers,respectively.First,the output current is significantly improved due to the high-density 3DEG.Second,the SPD-HFET utilizes 3DHG to block the vertical conductive channel between the source and 3DEG,thereby a new enhancement-mode operation is achieved.Third,the polarization-junction assists depleting the drift region in the off-state,optimizes the electric field distribution,and the source electrode forms a source field plate on the right side of the gate to further optimize the electric field of the drift region,thus the BV is drastically enhanced.Compared with the conventional structure,the BV of the SPD-HFET is increased from39 V to 919 V,and the saturation drain current is increased by 103.5%.In addition,the new structure also features low specific on-resistance(R_on,sp),large transconductance,and adjustable threshold voltage.2.To improve the BV and reduce the R_on,sp of vertical GaN HFET power devices,a vertical superjunction enhancement-mode AlGaN/GaN HFET?VSJ-HFET?is proposed.It is characterized by a superjunction voltage-sustaining layer composed of n-GaN/p-GaN.On the one hand,the alternating n-pillar and p-pillar in the new structure are depleted from each other in the off-state,which modulates the vertical electric field and makes it more uniform,thus the BV of VSJ-HFET is greatly enhanced.On the other hand,the depletion between the p-pillar and n-pillar increases the doping concentration of n-GaN drift region,thus a lower R_on,sp is achieved.Simulation results show that the relationship between the R_on,sp and BV of the new structure is significantly improved.The R_on,sp of VSJ-HFET is 0.86 m??cm²,which is decreased by 54%compared with the1.87 m??cm² of conventional vertical GaN HFET at the same 1800V-class BV.