The Influence Of Surface Treatment On AlGaN/GaN HEMT Electrical Performance

In recent years, Al GaN/GaN HEMTs displays its high performance in many ways, including high temperature,environment, high power and microwave application. However leakage currents still exists in GaN devices, it would cause additional current noise, current collapse effect and other reliability problems. So it is necessary to improve the reliability of GaN HEMT.According to our experiment, we mean to improve the performance of device by optimizing process parameters. We arrange different surface treatments on devices surface before preparing gate metal, including HF solution, HF/KOH solution, N₂ plasma/HF/KOH solution, HF/KOH solution/Al₂O₃. By comparing the performance of ohmic contact and mesa isolation with and without surface treatment, we conclude that: RSH decreases, Rc increases with the treatments including HF, KOH, HF/KOH, HF/KOH/Al₂O₃ solution treatment and Si Nx passivation treatment only. N₂ plasma contributes to the decrease of RSH and Rc. HF solution accompanying by KOH solution treatment can decrease the RSH significantly. As to ISO test, we perform HF solution, KOH solution and N₂ plasma respectively. We find that they are helpful to enhance the effect of the mesa isolation, and at same time N₂ plasma has the most significant effect.After fabrication process, we analyze the devices performance to find out the influence on electrical properties with different surface treatment. Maximum saturated output current decreases with HF solution, HF/KOH/Al₂O₃, HF/KOH solution treatment. There is no significant change on maximum saturated output current with N₂ plasma /HF/KOH, KOH solution treatment. In additional, N₂ plasma /HF/KOH contribute to the increase of transconductance mostly, with threshold voltage maintaining at-2.88 V.By comparing three terminal device leakage, CV leakage and surface leakage, we conclude that: surface treatment has little effect on the bulk leakage; HF/KOH solution can deteriorate the performance of the device, at same time other treatments can reduce the leakage; the N₂ plasma treatment can inhibit the reaction of the HF/KOH solution and the surface, so it can reduce the leakage current; The film of Al₂O₃ can reduce surface and mesa leakage significantly with currents decreasing from 1×10^-4A to 5×10^-6A?Vgs=-15V?. At last, we analyze the interface state in gate metal and barrier layer. The interface state is characterized by conductance method. By extracting the time constant and the density of interface states, we find that the range of time constant and the density of interface states decrease with HF solution, N₂/HF/KOH solution, KOH solution and HF/KOH solution treatment, which means some trap energy levels become shallower.