Mechanism And Reliability Study Of Novel High-voltage Low-loss β-Ga₂O₃ MOSFET

Monoclinic crystalline phase gallium oxide（β-Ga₂O₃）has excellent material properties such as ultra-wide bandgap of 4.5～4.9 e V and ultra-high critical breakdown electric field strength of 8 MV/cm,and the Baliga’s figure of merit for the device is 3444,10 and 4 times higher than that of Si,Si C and Ga N devices,respectively.This indicates that at the same breakdown voltage（BV）level,the length of the drift region ofβ-Ga₂O₃can be reduced to obtain a lower specific on-resistance(R_on,sp),which results in lower losses and smaller size.Therefore,β-Ga₂O₃ power devices have the advantages of low R_on,sp,high BV and low cost,especially suitable for high-power,high-energy efficiency and high reliability applications.However,to achieve higher performance devices and to further explore the theoretical limits ofβ-Ga₂O₃,four key issues need to be addressed as follows:（1）The BV ofβ-Ga₂O₃ devices is much lower than the theoretical limit,due to the lack of p-type doping,requiring the design of novel junction termination technology.（2）β-Ga₂O₃ MOSFETs are not able to simultaneously combine high threshold voltage(V_TH),low R_on,sp,high BV and high current density.（3）Theβ-Ga₂O₃ devices are subjected to electrical,thermal and coupled stresses with unknown degradation mechanisms,missing degradation models and lack of corresponding reliability enhancement technology.To address the above key issues,two novel device structures and two degradation models are proposed in this article from the whole chain ofβ-Ga₂O₃ MOSFET structural design,simulation study,fabrication process,test characterization and reliability analysis to conduct device innovation research,and the main innovations are as follows:1.Proposing a novel structure for high-voltage low-power enhanced gate-controlled heterojunction verticalβ-Ga₂O₃ MOSFETThis work presents new design concepts for realizing high-voltage low-power verticalβ-Ga₂O₃ MOSFET.By forming a vertical MOSFET（GCH-VMOS）with a gate-controlled NiO/β-Ga₂O₃ heterojunction（GCH）on one side of the gate trench and a Metal-Insulator-Semiconductor（MIS）on the other side.This structure combines excellent properties such as high V_TH,high saturation current density and high BV with high manufacturing feasibility and low process accuracy demands.Under zero bias,the GCH and the MIS structure work together to deplete the channel,resulting in enhancement;during blocking,the GCH is in reverse bias,which enhances the pinch-off effect on the channel,resulting in low leakage current and high BV;during on-state,the MIS structure accumulates electrons,and the positive bias of the GCH contributes to the expansion of the conducting channel,which results in high current density.The simulation results realize enhanced GCH-VMOS V_TH=0.6 V,R_on,sp=4.26 mΩ·cm²,maximum saturation current density of 1.5 k A/cm²,BV=2583 V,and power figure-of-merit of 1.54 GW/cm²,which are leading the current enhancement of verticalβ-Ga₂O₃ MOSFETs.2.Propose and study the interface dipole ionization model for NiO/β-Ga₂O₃heterojunction FET（HJ-FET）and electrothermal aging（ETA）technologyIn this work,an interface dipole ionization model is innovatively proposed to simultaneously explain the NBS-induced thinning in the space charge region and the decrease in the recombination centers concentrations,which leads to negative/positive shifts in the electrical characteristics of the HJ-FET and the gate-source equivalent diode,respectively.Based on this model the proposed ETA technology is evaluated at high temperature NBS and found that the reliability of the device is substantially enhanced.At40℃,the V_TH shift ratio after ETA treatment is significantly reduced to 0.4%from 31%before treatment.In addition,it is found that the ETA-treated device also maintains an extremely low V_TH shift ratio of 2.4%at 200℃.The discovery of ETA technology forβ-Ga₂O₃ heterojunction high-reliability becomes possible,significantly promoting the commercialization ofβ-Ga₂O₃ power devices.3.Investigation of degradation mechanism ofβ-Ga₂O₃ MOSFETs under electro-thermal stress and proposing an ionized traps modelIn this work,a measurement-stress-measurement method combined with hysteresis testing is used to innovatively investigate the V_TH instability mechanism ofβ-Ga₂O₃MOSFETs under gate stress and thermal stress.Under positive gate bias stress（PBS）,the electron captured by the border traps in the gate dielectric Al₂O₃ leads to a positive V_THshift,and as the temperature increases,the activated deep-level acceptor-type interface states and border traps capture more electrons leading to a more severe positive V_TH shift in the device.Unlike PBS,a combination of border traps,interface states and bulk traps under negative gate bias stress（NBS）leads to the device performance degradation.The ionization trap model is proposed to explain the non-uniform degradation mechanism of the device under PBS and NBS,and the accuracy of the model is verified by the simulation and experimental consistency.The degradation model created in this work can distinguish between interface states and border traps,which help to identify defects and optimize the device structure and fabrication process to provide an important theoretical foundation for improving the reliability ofβ-Ga₂O₃ MOSFETs in high-power applications.4.Proposed and experimentally realized a novel structure of high BV 2nd-order gate-drain floating field-plateβ-Ga₂O₃ MOSFETIn this work,β-Ga₂O₃ MOSFET with various types of field plates are fabricated with innovative layout design and high precision process control,focusing on the effect of the field plates on the breakdown characteristics.The 2nd-order gate-drain floating field plate effectively optimizes the electric field distribution at the gate and drain during reverse blocking,and its BV reaches a maximum value of 1583 V,which is 362V higher than the BV of the conventional field plateβ-Ga₂O₃ MOSFET,and this provides some theoretical guidance for the research ofβ-Ga₂O₃ MOSFET field plate technology.