Study On High-Electric-Field Degradation Effects And Thermal Issues Of Gan-based HEMT Devices

High electric-field degradation effects and thermal issues of GaN based highelectron mobility transistors (HEMTs) are investigated in this dissertation.Firstly, high electric-field degradation effects of GaN based HEMT areinvestigated by designing of constant stress-recovery and step stress-recovery schemes.Comparison of DC characteristics, light recovery characteristics and trapping-releasebehavior of the device shows that the dominant degradation mechanism under thespecific on-state stress differs from that of the off-state stress condition. Off-state drainstep stress, VDS=0gate step stress and on-state drain step stress are applied respectivelyto investigate the critical behavior of device under high electric-field. A coupled modelbased on the electron capture mechanism (hot-electron-driven) and the inversepiezoelectric polarization mechanism (electric field driven) is proposed to explain theobserved device degradation. The distinction of the two leading electrical degradationmechanisms is achieved by switch of gate leakage current behavior. The electroncapture mechanism plays a leading role when the device is biased at high output power,while in the case of high-field (Off state, VDS=0V, VGS<0V) the inverse piezoelectricpolarization effect is believed to be dominant.High-temperature characteristics of AlGaN/GaN HEMTs including ohmic contact,Schottky contact, DC and AC characteristics are studied systematically and possibledegradation mechanisms are proposed to explain the various degradation phenomenon.The specific contact resistance keeps constant as temperature increases, which showsexcellent stability of the ohmic contact metallization under high temperatureenvironment. Trap-assisted Frenkel-Poole (FP) emission model can explain a significantincrease in gate leakage current at higher temperature and get reasonable physicalparameters. The saturation drain current and the peak trans-conductance decrease withthe increasing temperature, which is mainly due to the reduction in the2DEG mobilityand the decrease in the2DEG density. Our study on the conductance-frequency and thegate-lag characteristics reveals that no new trap is generated in the AlGaN/GaN HEMT,while the activity of the original trap in the AlGaN barrier is enhanced as thetemperature increases, which contributes to the electron escaping from the traps in thebarrier layer, thus suppresses the depletion effect of the trapped electrons to the channeland eventually leads to the weakening of the current collapse. The thermal storage reliability of AlGaN/GaN HEMTs is investigated fromcapacitance, DC, AC and frequency characteristic aspects. It is found that thermalstorage can reduce the effective thickness of the AlGaN barrier layer. Through thestorage of250℃and96hours, the effective thickness of the AlGaN barrier decreasesfrom25.7nm to22.8nm. The density and time constant of interface state traps ofdevices before and after thermal storage are obtained by Gp-ω calculation. It can be seenthat the interface state density is reduced significantly, and explain the improvement ofthermal storage on device interface characteristics. More ideal and intimate the Schottkycontact is formed due to high temperature storage, while removing the interface statedensity beneath the gate, thus reduce the gate leakage current increase the breakdownvoltage. Current collapse and gate-lag characteristics are suppressed after storage, whichimplies the inhibitory effect of thermal storage on the interface states. In addition,thermal treatment has no significant effect on the frequency characteristic of device.Research on the multi-finger devices shows that the principles of thermal storage ondevice with100μm gate width also applies to the multi-finger devices. Finally, thethermal storage condition is optimized,250℃and24hours heat storage plays the roleof the curing device and can effectively reduce the device parameter drift in thefollow-up work.By excluding the inverse piezoelectric bias, micro-Raman spectroscopy issuccessfully employed to measure the channel temperature of multi-finger AlGaN/GaNHEMT under operation. Then, the junction to case thermal resistance is calculated,compared to that of devices reported in current literature, our device has reached anadvanced level (Rjc=2.62℃/W). Research on the horizontal and vertical temperaturedistribution through the device enhances our understanding on the heat generationlocation and the transmission mode, which is of great importance for predicting thelifetime and to optimize the layout structure, and thus alleviate the self-heating effect inthe multi-finger microwave power devices.A coupled electro-thermal model is constructed to describe detailed electrical andthermal behavior of multi-finger AlGaN/GaN HEMTs in this paper. The model takesthe2DEG transport characteristic, the Joule heating power distribution and thenonlinear relationship of material thermal conductivity with temperature into account.The results reveal excellent correlation to the micro-Raman measurements, validatingour model for the design of better cooled structures. Furthermore, the influence oflayout design on the channel temperature of multi-finger AlGaN/GaN HEMT is studied using the proposed electro-thermal model, results indicates that parameters such as thenumber of fingers, finger width, and spacing are critical factors for thermal managementconsiderations and reasonable optimization methods for layout design are suggested toeliminate the self-heating effects.