AlGaN/GaN HEMT Reliability

As the third generation semiconductor material,gallium nitride(GaN)has excellent properties such as wide direct energy gap,high breakdown field strength,high electron mobility,good heat resistance and good radioresistance.So it is very suitable for high temperature,high frequency,high power and high breakdown voltage power electronic devices.AlGaN/GaN HEMT power electronic devices is based on two dimensional electron gas at AlGaN/GaN heterojunction,and have become a research hotspot at the present stage and show great potential for application.However,there are many problems,especially the reliability,which restricts the large-scale application of AlGaN/GaN HEMTs.Therefore,the reliability of AlGaN/GaN HEMTs is one of the research focuses,and it is also an urgent technical problem to be solved at present.In this thesis,the research was mainly focused on the characteristics and reliability of AlGaN/GaN HEMTs.Based on structural design and theoretical analysis,we optimized the fabrication process of the device and analyzed its reliability and physical mechanism.The main contents of this thesis are as follows.1.The leakage and breakdowm characteristic of AlGaN/GaN HEMTs were investigated.We studied the path of isolation leakage by ion implantation.It was found that the interface between AlGaN and SiNx is the main leakage path in the ion implantation area.An annealing post-implantation process could reduce the interface state density,and therefore reduced the leakage of the device with an on/off ratio of 1010.In addition,we fabricated MIS-HEMTs by inserting an LPCVD SiNx layer as gate dielectric under the gate electrode and employing the field plate structure to modulate the electric field.The breakdown voltage of the devices could reach 1082 V with 7 ?m gate-to-drain space.However,the trend of dynamic characteristics with field plate length was opposed to the breakdown voltages,and we should make a trade-off between the two aspects.2.The influence factors and reliability on ohmic contact of AlGaN/GaN HEMTs were investigated.The reliability of ohmic contacts could be affected by the fabrication conditions.We studied the effects of surface treatment and Ti/Al ratio,thickness of Au,ICP etching and annealing temperatures on the ohmic contact resistance,and obtained ohmic contact resistance as low as 0.24 ?·mm employing 20/130/50/50 nm Ti/AI/Ni/Au annealed at 810 ? after ICP etch(removing 12 nm GaN caplayer and AlGaN barrier),5 min 200 W O plasma treatment and 2 min HCl:H2O=1:10 treatment.Besides,the sample showed a better surface morphology and better stabilities measured at the elevated temperatures and after the long-term aging at 400 ? N2 ambient comparing with other samples.3.The dynamic characteristics,interface states and the dielectric of AlGaN/GaN HEMTs were investigated.We fabricated MIS-HEMTs by inserting an LPCVD SiNx layer as gate dielectric,and suppressed the current collapse by increasing the flow ratio of SiH2Cl2 and NH3 in the LPCVD SiNx growth process.Through the interface state analysis and SIMS measurement,we inferred that the physical mechanism is Si atom incorporation at the surface.The Si atoms act as donors,and the deep-level traps will be filled by electrons ionized from Si donors,leaving positive charges.Then,the leaving positive charges of the shallow Si donors reach the charge neutrality with 2DEG and the negative polarization charges,thereby screening the deep-level traps and suppressing the virtual gate effect.We designed a new device fabrication process employing LPCVD SiNx as the dielectric under the field plate,which can improve the breakdown voltage and suppress the current collapse more effectively.The normalized dynamic on-resistance could achieve 1.18 after 600 V off-state drain voltages.4.The long-term degradation of AlGaN/GaN HEMT was investigated.We set up a long-term off-state stress degradation test system for AlGaN/GaN MIS-HEMTs.In the long-term degradation,the devices exhibited decreased gate leakage current,periodically increased and recovered drain leakage,left shifted threshold voltage,increased hysteresis,increased on-resistance,and decreased output current.Through the analysis of these phenomena and the left drift of the E2 peak on Raman spectra under off-state stress,we believed that the inverse piezoelectric effect is the main physical mechanism in the long-term off-state stress degradation of AlGaN/GaN MIS-HEMTs.