Study On Defects In High-Filed Of GaN-based HEMT Devices

As the third-generation semiconductor material,Gallium Nitride(GaN)has numerously superior properties such as high electron mobility,high breakdown field strength,high temperature resistance and good radiation resistance.Therefore,Gan-based HEMT devices are well suitable for the next generation 5G communication,national defense,military,aerospace and other special fields.However,there are some reliability problems in the GaN-based HEMT devices,which restrict the commercial application of GaN-based HEMTs.The external environment,such as electric field,temperature,irradiation,and etc.,is the external cause of GaN-based HEMT device degradation.Besides,the inherent trap behavior is the internal cause of GaN-based HEMT device degradation.In this paper,the research was mainly focused on the degradation and defects of Depletion-mode HEMT devices and MIS-HEMT devices under electric stress.Firstly,the on-state stress,off-state stress,forward and reverse bias experiments were carried out on the Depletion-mode HEMT devices.The experimental results show that there is no obvious hot electron effect in the early stage of the on-state stress.There is only an electron de-trapping process caused by the electric field,which lead to the negative shift of the threshold voltage.Subsequently,hot electron effect was starting to become apprent.Hot electron was trapped in AlGaN barrier,which can counterbalance part effects of de-trapping,and the threshold voltage would shift in the positive direction.At the same time,hot electron effect leads to the decrease of transconductance due to the creation of new interface states.By changing drain voltage,device size and the condition of voltage bias,different off-state stress experiments are carried out on the device.The negative shift of threshold voltage after off-state stress in all of devices could be explained by the process of de-trapping.The same device size,high drain voltage,and the same drain voltage and small device size cause the device to be in a strong electric field.At this time,the reverse piezoelectric effect of the device creates a new trap,which is represented by the increase of gate leakage current.It indicates that the high electric field caused by high voltage or small size of the device is the decisive factor leading to the inverse piezoelectric effect.Then,keeping the gate-drain voltage difference,the gate leakage current increases only after the negative stress is applied to the gate,it has indication that the damage caused by the gate stress to the device is higher than the drain stress.Forward and reverse bias experiments have proved that the primary traps in AlGaN barrier layer are mostly full state.And the process of de-trapping is extremely easy to take place under electric field,which leads to the negative shift of threshold voltage.Then,the forward and reverse bias stress experiments were carried out in MIS-HEMT devices.It is determined that the primary as-grown traps in MIS-HEMT device are Si3N4 dielectric layer/AlGaN barrier layer interface state traps.After forward bias stress,the electrons will fill the interface state traps.If a high voltage is applied to the drain,a large amount of hot electrons will also fill the interface state traps,so threshold voltage of the device would shift in the positive direction.If there is a strong trap-assisted tunneling or leakage path in the dielectric layer material,threshold voltage would also shift in the positive direction due to trapping of electrons at the dielectric–semiconductor interface.