Improvement of the surface of buffer region of AlGaN/GaN heterostructure by employing the SiO₂ deposition was investigated. We deposited the SiO₂ layer on the patterns and removed the SiO₂ layer by wet-etching process. After the removal of SiO₂ layer, the electrical characteristics of active region were almost identical to those measured before the deposition of SiO₂ layer. However, the buffer leakage current was decreased by 4-orders after the removal of SiO₂ layer. This may be attributed that the treatment on the surface of buffer region during the process of the deposition and the removal of SiO₂ layer.