Reliability Mechanism Related To Defects In GaN-based Power Electronics Devices

Owing to the high breakdown voltage,high electron density,high-saturated electron velocity and high carrier mobility,Gallium Nitride High Electron Mobility Transistors?GaN HEMTs?have recently emerged as highly attractive candidates for the next generation high efficiency power electronic system.For fail-safe operation and simplified gate drive topology,normally-off operation modes are demanded in power switching applications.Consequently,two gate topologies are the mainstream to achieve normally off operation in GaN based devices,i.e.,metal-oxide-semiconductor Heterostructure Field Effect Transistors?MOSHFETs?and p-type gate GaN HEMTs.The p-GaN HEMTs are currently on track to become the first Enhance-mode?E-mode?power devices to be commercialized.However,the metal/p-GaN/AlGaN/GaN gate stack creates a new gate structure that differs from the classical largely adopted MOS structure in Si-based technologies.Consequently,an extensive gate reliability characterization is strictly required to understand the possible mechanisms limiting the device reliability.On the other hand,the high-density traps at the dielectric/III-N interfaces and GaN buffer layer bring about the threshold instability and gate dielectric reliability issues that hinting the commercialization of GaN MOSHFETs.It is therefore important to study the trap distribution in the GaN MOSHFETs.The thesis has investigated the gate reliability of the E-mode GaN HEMTs-on Silicon in view of the high voltage operation of GaN power devices.Starting from the unique energy band structure and space charge distribution of the metal/p-GaN/AlGaN/GaN gate stack,an electron-trapping and hole-injection model was proposed to unveil the threshold instability of the p-GaN HEMTs.Then,for the fist time,the gate degradation mechanisms of the p-GaN HEMTs were also investigated under long-term stress and recovery method.Moreover,a depletion-region conductance method was proposed to differentiate the interface and bulk traps in the normally-off GaN MOSHFETs,which leads to the instability and reliability issues in the GaN MOSHFETs.In particular,the thesis provides the following contributions:1.Electron-trapping and hole-injection model in the p-GaN HEMTsThe comprehensive voltage dependent,time-resolved and temperature-dependent gate current characterization experimentally reveals the intricate carrier transport dynamics through the gate stack of the p-GaN HEMTs.The detailed carrier transport dynamics,the sequence in the metal/p-GaN/AlGaN/GaN gate stack as well as the applied gate bias have been well captured by the transient gate current characteristics.Moreover,the specific trap energy levels involved in the carrier transport through the p-GaN gate stack were revealed by the temperature dependent gate current analysis.These facilitate the understanding of gate carrier conduction and hence the V_THH instability of the p-GaN HEMTs.The gate current is decomposed into electron and hole current in three distinct regions with respect to V_G,which are the off-state region for V_G<V_TH,the on-state region for V_TH<V_G<V_GT and the“gate-injected”region for V_G>V_GT.In off-state,the electrons were thermally activated and transport towards the gate,while thermally active holes are negligible.Electron-trapping in the AlGaN barrier governed by the Space Charge Limited Conduction?SCLC?and thermally active holes constitute the gate current in on-state,the former dominating before the channel saturation with V_TH<V_G<V_SAT and the later is the main part for V_SAT<V_G<V_GT.However,for V_G>V_GT,a drastic hole injection triggered by high V_G takes place that causes subsequent hole-trapping in the AlGaN barrier and hole-injection into GaN buffer.Hence the gate current consist the electron-hole recombination current and the SCLC-limited hole current.Electron-trapping in the AlGaN barrier enhances negative charges in the gate region,leading to the positive V_TH shift for V_G<6 V.While for V_G>6 V,a drastic hole injection triggered by high V_G takes place that causes subsequent hole-trapping in the AlGaN barrier and hole injection into GaN buffer.The injected holes enhance the positive charge in the gate region and turn the positively V_TH shift into a negative shift.2.The degradation mechanisms of the p-GaN HEMTsThe physical degradation mechanism in the gate region plays a important role in the long-term stability of p-GaN HEMTs.Taking account of the slow time-constant property of hole-trapping and detrapping in GaN material,the gate reliability of the p-GaN HEMTs has been firstly investigated in the long-term recovery duration after long-term forward gate stress.Two different failure mechanisms were experimentally identified in the gate region when the p-GaN HEMTs were submitted to the long-term forward gate stress.One is the nondestructive and self-recovery failure of the AlGaN barrier in the recovery period induced by the trap-creation by the collision of AlGaN lattice with massive injected holes.The other is the degradation and breakdown of the metal/p-GaN Schottky barrier under high Electric field.Both of the above two mechanisms contribute the gate degradation in p-GaN HEMTs on long-term high positive gate stress.Besides,the degradation of AlGaN/GaN heterojunction was also observed when the gate p-GaN/AlGaN/GaN diode sustains the high gate voltage after the breakdown of metal/p-GaN Schottky diode.3.A depletion-region conductance method in GaN MOSHFETsIt is very hard to differentiate the interface traps and bulk traps in GaN MOSHFETs.Base on the energy band structure in the MOS gate stack,the interface traps and bulk traps of the GaN MOS Capacitors?MOSCAPs?were simply discriminated by extending the applied gate voltage range of the conventional conductance method.Moreover,the trap distribution can also be obtained through comparing the simulated and experimental transfer curves of the GaN MOSHFETs.Owing to the large long and wide ratio,the sidewall traps are negligible in GaN MOSHETs.Hence the trap densities extracted from the transfer curves of GaN MOSHFETs are much greater than that calculated from the GaN MOSCAPs.