Study On AlGaN/GaN MOS-HEMT With High Dielectric

Thanks to the fast development of science and technology,the requirements for electronic devices in various industries are increasing.Traditional Si-based semiconductor devices are difficult to meet the needs in some special applications.With the advantages of high breakdown field strength,high thermal conductivity and high electron mobility,Gallium Nitride have attracted attention of researchers.As a result,GaN HEMT electronic devices have become one of the hotspots of semiconductor devices.AlGaN/GaN HEMT devices with traditional Schottky gate electrodes suffer from high gate leakage current,small gate swing and poor breakdown characteristics.MOS-HEMT devices with high k dielectric are used to improved these characteristics.However,the introduced interface of dielectric/AlGaN has increased the complexity of the system.Interface defects,traps and bulk charges would cause bad performances such as negative shift and instability of threshold voltage and current collapse.Consequently,the key point turns out to be appropriate gate dielectric materials and process selection.This work mainly studies MOS-HEMT devices with Si compound gate dielectric deposited by plasma enhanced vapor deposition（PECVD）and the Al₂O₃ gate dielectric deposited by magnetron sputtering.The main contents are as follows.1.The process condition of PECVD deposited silicon oxynitride（SiON）are optimized.The optimized thin film has a relative dielectric constant of 6.27 and a breakdown field strength of 9.69MV/cm.Three MOS-HEMTs of different dielectric layers,SiN_x,SiON and SiO₂ were fabricated.Among the three samples,SiO₂ MOS-HEMT obtains the lowest gate leakage current,SiON MOS-HEMT obtains the highest breakdown voltage of 428V.The density and distributions of interface states were analyzed by CV measurement and transfer curves pulse test.The interface states of SiN_x MOS structures are mainly distributed in relatively shallow levels,while the interface states of SiON and SiO₂ MOS structures are mainly concentrated in deep levels.The deep-level interface states play a major role in current collapse.The SiN_x MOS-HEMT device performs best in terms of interface quality,which has the lowest current attenuation（11.6%）after the off-state drain voltage of 50V.2.The processed conditions of magnetron sputtering deposition of Al₂O₃ were optimized,which leads to a breakdown field strength of 9.1 MV/cm.The damage of AlGaN/GaN epitaxy by sputtering process was studied.The damage was repaired by annealing process and the current attenuation was reduced from 65%⁷0%to 5%.The preparation of MOS-HEMT device is completed by using two different processes.One is gate dielectric first followed by passivation layer fabrication,the other is passivation layer first and followed by gate dielectric layer fabrication.A vacuum heat treatment process were used to improves the negative effects of gate trench etching.The information of interface states are extracted by CV measurement and pulse test measurement.Great result of off-state drain voltage of 200V was observed.The saturated current slightly increases instead of decreases after applying off-state voltage stress.