A buffer structure made of two different AlGaAs layers for the GaAs-based field-effect transistors grown by metalorganic vapor phase epitaxy (MOVPE) has been demonstrated. The two AlGaAs layers that are grown in different conditions were developed to provide the demanded quality for buffer structure. The first AlGaAs layer on the substrate was grown at relatively low temperature to incorporate oxygen atoms. Thereby, the oxygen ions form the deep impurity level in the AlGaAs layer and this layer shows a high resistivity. By adjusting the growth condition, the second AlGaAs layer with high purity was deposited. To effectively suppress the leakage current between two adjacent devices formed on the two-AlGaAs-layer buffer structure has been proved. By using the two-AlGaAs-layer buffer structure and optimizing the growth condition, we have obtained the typical InGaAs/AlGaAs pseudomorphic high electron mobility transistors (PHEMTs) with sharp pinch-off characteristics and low output conductance. In addition, the minimization of side-gating effect has been achieved. (c) 2005 Elsevier Ltd. All rights reserved.