Long Term Stability of Enhancement Mode GaN Power Devices

GaN-on-Si technology, AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates, shows the promising characteristics and cost-competitiveness of power switching applications. In spite of the extraordinary performance and cost advantages, AlGaN/GaN HEMTs are still limited by their instabilities. For power-switching applications, GaN power devices operate at a high drain voltage during an OFF-state and at a high gate voltage during an ON-state, where good reliability is essential for these operating conditions. This dissertation focuses on the physical degradation mechanisms in the gate region that play a role in the long-term stability of Au-free enhancement-mode GaN power devices, especially for the two most important architectures: recessed gate Metal- Insulator-Semiconductor (MIS)-HEMTs/-FETs (Field-Effect Transistors) and p-GaN gate AlGaN/GaN HEMTs. Forward gate bias time-dependent dielectric breakdown (TDDB) and positive bias temperature instability (PBTI) are observed on depletion mode MIS-HEMTs and enhancement-mode MIS-FETs. The percolation model and Weibull distribution are used to understand the degradation mechanisms of forward gate bias TDDB, further calculating the lifetime. Regarding the PBTI, different techniques, i.e. a forward-reverse IDVG sweep, a frequency-dependent conductance method, and an AC-transconductance, are used to characterize the threshold voltage (VTH) hysteresis, interface states density (Dit), and the amount of border traps in the devices with different gate dielectrics. Furthermore, an eMSM (extend-Measure-Stress-Measure) method is used to study the stress-recovery phenomena in fully recessed gate MIS-FETs. A physical model, which can nicely reproduce the experimental data, is proposed to explain the origin of PBTI. Regarding p-GaN gate AlGaN/GaN HEMTs, temperature dependency of the forward bias gate breakdown is observed and characterized. Then, a physical model is proposed to explain the phenomenon. Furthermore, forward gate bias time-dependent p-GaN gate breakdown and positive bias temperature instability (PBTI) are also studied in p-GaN gate AlGaN/GaN HEMTs. The possible mechanisms are proposed to explain the time-dependent p-GaN gate breakdown phenomenon and a negative VTH shift under a positive gate bias.1 Introduction 1 1.1 The birth of GaN-based devices 1 1.2 Material properties 2 1.2.1 Comparison of semiconductor materials 2 1.2.2 Crystal Structures 3 1.2.3 Substrates 5 1.2.4 AlGaN/GaN heterostructure and two dimensional electron gas (2DEG) 6 1.3 Device architectures and processes 14 1.3.1 AlGaN/GaN High Electron Mobility Transistors (HEMTs) 14 1.3.2 AlGaN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors (MIS-HEMTs) 15 1.3.3 E-mode AlGaN/GaN-based technologies 16 1.3.4 Imec’s Au-free CMOS-compatible E-mode technologies 20 1.4 Overview of reliability issues in GaN power devices 22 1.4.1 Introduction 22 1.4.2 OFF-state related reliability issues 22 1.4.3 SEMI-ON state related reliability issues 28 1.4.4 ON-state related reliability issues 29 1.4.5 Conclusion 31 1.5 Thesis content overview 31 1.5.1 Thesis objective 31 1.5.2 Thesis outline 31 2 Methodologies for exploring reliability issues under the gate 33 2.1 Introduction 33 2.2 Time-dependent dielectric breakdown (TDDB) experiment 33 2.2.1 Time-to-breakdown 33 2.2.2 Statistical properties and data analysis 36 2.2.3 Lifetime extrapolation 41 2.3 Interface characterization methods 44 2.4 VTH shift and Positive Bias Temperature Instability (PBTI) characterization 49 2.4.1 VTH shift characterization 49 2.4.2 Positive Bias Temperature Instability (PBTI) characterization 50 2.5 Summary of this chapter 55 3 Stability of D-mode AlGaN/GaN MIS-HEMTs on a 150mm Si substrate 57 3.1 Introduction 57 3.2 Device Description: AlGaN/GaN MIS-HEMTs with in-situ SiN/Al2O3 58 3.3 Typical characteristics 59 3.4 Forward gate bias time-dependent dielectric breakdown (TDDB) 61 3.5 Forward gate bias stress 64 3.6 Summary of this chapter 66 4 Stability of E-mode recessed gate GaN MIS-FETs on a 200mm Si substrate 69 4.1 Introduction 69 4.2 E-mode AlGaN/GaN transistors with the recessed gate process 69 4.2.1 GaN-on-Si epitaxy substrates in this study 69 4.2.2 Recessed gate process and gate dielectrics used in this study 70 4.3 The impact of gate dielectric quality on the output drain current 71 4.3.1 Evaluation of the gate dielectric quality in recessed gate D-mode MIS-HEMTs 71 4.3.2 The impacts of gate dielectric quality on the output drain current in E-mode fully recessed gate MIS-FETs 76 4.4 Stability of forward gate bias time-dependent dielectric breakdown (TDDB) 81 4.4.1 Gate width scaling 85 4.4.2 Percolation path in fully recessed gate E-mode MIS-FETS 87 4.4.3 Lifetime extrapolation in fully recessed gate E-mode MIS-FETS 87 4.4.4 Conclusion 89 4.5 Towards Understanding Positive Bias Temperature Instability (PBTI) in fully recessed Gate GaN MIS-FETs 90 4.5.1 ID-VG characteristics and interface characterization 90 4.5.2 Insight into PBTI mechanisms 92 4.5.3 conclusion 103 4.6 Summary of this chapter 104 5 Stability of E-mode p-GaN AlGaN/GaN HEMTs on a 200mm Si substrate 107 5.1 Introduction 107 5.2 Device Description 107 5.2.1 The forward gate bias breakdown mechanisms 109 5.2.2 Conclusion of the forward gate bias breakdown mechanisms 113 5.2.3 The high forward gate bias stress in p-GaN AlGaN/GaN HEMTs 114 5.2.4 Conclusion of the high forward gate bias stress 120 5.3 Summary of this chapter 121 6 Summary and outlook 123 6.1 Main Objective 123 6.2 Summary of this Ph.D. work 123 6.3 Outlook 126 Bibliography 129 Scientific Contributions and Awards 143 Curriculum Vitae 149status: publishe