High electron mobility transistors（HEMTs）show tremendous potential for high mobility, high breakdown voltage, low conduction, low power consumption, and occupy an important piece of the microelectronics field. High-resistivity-cap-layer high electron mobility transistor (HRCL-HEMT) is a novel device structure. Based on the hole compensation mechanism, the p-GaN is converted into high resistance semiconductor material by hydrogen plasma implantation. Thus, the surface of the p-GaN layer will have a serious bombardment damage under the hydrogen plasma implantation. In practical work, it is also very challenging in the accurately control of the hydrogen injection rate, injection depth and injection uniformity. To achieve the required depth of injection, the energy of hydrogen plasma is often injected in more than the required dose or multiple injections times. The energy of hydrogen plasma plays a huge influence on the surface of the p-GaN layer. leakage current will be generated on the device surface, which deteriorate the electrical performance of the device. In this work, to protect the surface of p-GaN layer, a 2 nm Al2O3 film was deposited on the surface of the p-GaN cap layer to reduce the implantation damage caused by hydrogen Plasma treatment. The research shows that after the device deposited Al2O3 film before hydrogen Plasma treatment, the gate reverse leakage current was reduced by an order of magnitude, the ratio of ION and IOFF was increased by about 3 times. Meanwhile, the OFF-state breakdown voltage was increased from 410 V to 780 V. In addition, when the bias voltage was 400 V, the dynamic RON of devices A and B were 1.49 and 1.45 respectively, the device B showed a more stable dynamic performance. To analyze the gate leakage mechanism, a temperature-dependent current IG-VG testing was carried out, and it was found that the dominant mechanism of gate leakage current was Two-dimensional variable range hopping (2D-VRH) at reverse gate voltage. The reason for reducing the gate reverse current was analyzed, the Al2O3 film increases the activation energy of trap level and changed the surface states of HR-GaN; Furthermore, Al2O3 film blocked the injection of too much H plasma, reduced the density of AlGaN barrier and channel trap states, and weakened the current collapse.

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