Study On The Traps And Reliability Of GaN-based HEMTs

GaN-based HEMTs have a great advantage in material and device structure.However,the reliability still limits the further marketization of GaN-based HEMTs,and many reliability problems are related to traps.In this dissertation,the special phenomena were studied under high-field stress,and the corresponding physical degradation mechanisms were proposed.The traps in AlGaN/GaN heterojunction were detected by deep level transient spectroscopy.In this dissertation,the material advantages of GaN and the huge potential of GaN-based devices in the application of microwave devices and power electronics were introduced.The current reliability problems of GaN-based devices were summarized,including inverse piezoelectric effect,electrochemical reaction,hot carrier effect,threshold voltage stability,current collapse and dynamic on-resistance degradation.The traps and their impact on characteristics and reliability of GaN-based HEMTs were discussed.The deep level transient spectroscopy of AlGaN/GaN heterojunction was investigated.The Ec-0.56e V trap was detected in the GaN layer,which was determined as a point defect(nitrogen antisites N_Ga).Ec-0.88e V(nitrogen antisites N_Ga)and Ec-0.33e V(nitrogen vacancies V_N)traps were detected in the AlGaN layer.The surface state emitted and captured electrons,leading to the appearance of hole-like trap signals when the filling pulse width increased.The signal peak of the electron trap would decrease or even disappear when surface states dominated.Suppression of surface states was important not only to optimize the performance of devices but also to improve veracity of electron traps DLTS measurements in AlGaN/GaN heterostructure.The degradation of GaN-based HEMTs under gate reverse step stress at 77K was investigated.It was the first time to investigate the inverse piezoelectric effect at 77K,and to apply the ultraviolet light in this mechanism investigation.The experimental results indicated that it was the temperature rather than the ultraviolet light that have impact on the inverse piezoelectric effect.As the temperature decreased,it was more difficult for the inverse piezoelectric to occur,due to the lower tensile stress of the as-grown barrier layer at low temperature.A theoretical model of the inverse piezoelectric effect including temperature was established.The trapped electrons at 77K were difficult to be re-emitted,resulting in more positive drift of threshold voltage and more serious drain current degradation.The contributions of the inverse piezoelectric effect and trap effect to the degradation of the devices were analyzed quantitatively.It indicated that the inverse piezoelectric effect dominated on the degradation of the off-state characteristics,while having little effect on the maximum drain current and maximum transconductance.The traps in the barrier layer and at the Si N/AlGaN interface induced a positive drift of the threshold voltage,degradation of the drain current the maximum transconductance,which were more serious at 77K.The results were significant for the applications of GaN-based devices in extremely low temperature environments,such as deep space exploration.The abnormal threshold voltage drift of MIS-HEMT devices under reverse gate voltage stress was investigated.Through the study of the mobility during the stress and recovery process,the relevant traps were determined near the channel,and a double-trap-model was proposed(the traps at the Si N/AlGaN interface and the traps in the barrier layer).It was found that the two traps had opposite effects on the threshold voltage drift.During the stress process,the traps released electrons at the Si N/AlGaN interface,causing the threshold voltage to shift negatively.The traps in the barrier layer caused the threshold voltage to shift positively by Zener trapping.In the recovery process,the traps at the Si N/AlGaN interface were refilled with electrons to cause the threshold voltage to shift positively,while the traps in the barrier layer emitted electrons to the conduction band by the thermal emission,which caused the threshold voltage to shift negatively.The combined action of the two mechanisms induced in the complex threshold voltage drift phenomenon under different stress conditions.The degradation of Fin-HEMTs was investigated under off-state high-field stress,for the first time,with planar HEMTs as a reference.The electrical characteristics of Fin-HEMTs under this stress condition were exhibited,and the corresponding physical model was established.Compared with planar HEMTs,Fin-HEMTs have better threshold voltage stability and the transconductance and maximum current increased during the stress process.The changes in characteristics of Fin-HEMTs would recover after 150°C annealing.The electric field in the two structure devices under the off-state high-field bias condition was simulated and analyzed by Silvaco Atlas.Compared with the planar HEMTs,the electric field in the barrier under the gate was reduced in the Fin-HEMTs,due to the sidewall gate.As a result,the effect of donor traps in this region was weakened and the threshold voltage was relatively stable.The peak electric field at the gate edge was reduced,which caused the virtual gate effect to be suppressed.Therefore,the degradation of the drain series resistance could be ignored.What was more,the high electric field appeared near the sidewall in the buffer layer below the channel.The electrons were injected by sidewall-gate under high electric field to form hot electrons.The hot electrons assisted the negatively charged acceptor traps emitting electrons,leaving neutral traps,and decreasing the scattering to the channel electrons,which induced in the increase of transconductance and drain current.When the acceptors changed to be neutral,electric field increased,especially in the buffer,resulting in the increased gate leakage.By comparing the deep level transient spectroscopy of fin-HEMTs before and after the stress,the acceptor trap energy level was found to be E_C-0.71 e V,which was Fe related trap,further validating the physical model.