We investigated GaN buffer layers grown on Si substrates using a multi-wafer metalorganic vapor-phase epitaxy (MOVPE) system for 4-in five wafers (5 x 4 '') growth in order to obtain field-effect transistors (FETs) with high breakdown voltages. It was confirmed that GaN films with smooth surfaces and without any cracks were obtained. To obtain high-resistive GaN buffer layers, C-doping was carried out by controlling the V/III ratio and the growth pressure. The breakdown voltage of the buffer layer was strongly correlated with the C concentration. As a result, the breakdown voltage was over 800 V when the C concentration was about 8E18 cm(-3). We also fabricated a heterojunction FET (HFET) using an AlGaN/GaN heterostructure on a C-doped GaN buffer layer. The breakdown voltage of the FET in the off state was over 500 V and the maximum drain current of the FET was over 300 mA/mm. It was thus confirmed that an AlGaN/GaN HFET with a high-resistive buffer layer on a 4-in Si substrate could be obtained. (c) 2006 Elsevier B.V. All rights reserved.