Studies On Novel Normally-off P-GaN Gate HEMT Power Switching Devices

GaN-based high-electron-mobility-transistors(HEMTs)on silicon substrate have been promising candidates for future power switching systems due to their low cost,high switching frequency,high conversion efficiency,high operating temperature,and small system size.Normally-off p-GaN gate HEMT has won favor of industrial circles for its gate reliability.However,challenges still exist in the process of commercialization,such as breakdown voltage,threshold voltage,gate swing,current collapse,reliability,and yield.This dissertation proposed a novel fabrication technology for p-GaN gate HEMT—hydrogen plasma treatment,which is a planarization process and features a large process window.Besides,high performance direct coupling field effect transistor logic(DCFL)is also realized by this technology.The main contents of this thesis are as follows:1.A novel fabrication technology for p-GaN gate HEMT is proposed and demonstrated.With hole compensation mechanism,normally-off p-GaN gate HEMT(HRCL-HEMT)with a threshold voltage of 1.75 V,a maximum drain current of 188m A/mm,and an on/off ratio of?10⁷,has been realized by hydrogen plasma treatment for the first time.Then,device performance has been improved a lot after detailed process optimization and plasma damage repair by rapid thermal annealing.The maximum drain current is about 350 m A/mm.Also,the optimized process has been demonstrated with a large process window of 4 min at least.In addition,the research results imply that the HRCL can effectively enhance breakdown voltage and suppress current collapse of p-GaN gate HEMT.2.Studies on gate leakage mechanism and off-state breakdown mechanism of the HRCL-HEMT are carried out.Gate leakage mechanisms of HRCL-HEMT with Ti as the gate metals was researched by I-V-T measurement.The result indicates that two-dimensional variable range hopping(2D-VRH)is responsible for gate leakage current at reverse and low positive gate bias,while the main gate leakage mechanism at high positive gate bias is Poole–Frenkel emission(PFE).In addition,the breakdown mechanism of HRCL-HEMT with substrate grounded and substrate floating have been analyzed.It is found that the breakdown voltage of HRCL-HEMT with substrate grounded is limited by the vertical leakage.The enhanced breakdown voltage in substrate floating case is due to the form of space charge region in the buffer between source and substrate.3.Thermal stability and gate reliability of HRCL-HEMT are studied.At first,the thermal stability of HRCL-HEMT is analyzed by high-temperature test and long-term thermal stress test.The devices keep good performance even at 200°C and no obvious performance degradation is observed during the 1000 h thermal stress test at 150°C.Then,gate lifetime of HRCL-HEMT is evaluated by time-to-failure(TTF)measurement.Also,step stress measurements have been carried out to analyze failure mechanism of gate.4.High power HRCL-HEMT is investigated.The 50 mm gate width HRCL-HEMT with TO-220 package is fabricated successfully by hydrogen plasma treatment.The fabricated device exhibits a positive threshold voltage of 2.3 V,a low threshold voltage hysteresis of 45 m V,a high drain current of 13.7 A at V_GS=6 V and V_DS=10 V,a high on/off ratio of?10⁹,a high breakdown above 650 V,and low capacitance parameters.5.GaN power integrated circuits are investigated.Monolithic integration of enhancement/depletion-mode HEMTs are realized by hydrogen plasma treatment.The fabricated GaN direct coupled FET logic(DCFL)shows large logic swing,wide noise margins,ultra-low threshold hysteresis,and good temperature stability.The 11-stage ring oscillator exhibits an oscillation frequency of 56.8 MHz and a propagation delay of 0.8ns/stage.In the aspect of GaN CMOS,a novel device structure of n-GaN gate p-channel device is proposed.The simulation result indicates that the n-GaN gate can realize normally-off operation without sacrificing the saturation drain current.