High breakdown voltage (BV) AlGaN/GaN heterojunction field effect transistors (HFETs) with a low specific on-resistance (AR(DS(on))) were successfully fabricated using intentionally C-doped semi-insulating GaN buffers with a high resistivity on sapphire substrates. With the improvement of not only the resistivity of a C-doped GaN buffer but also the layout design near the gate feeding region, the fabricated devices exhibited a high BV of similar to 1600 V and low AR(DS(on)) of 3.9 m Omega cm(2). This result even reaches the 4H-SiC theoretical limit and the best ever reported for the high-power GaN-based HFETs realized on sapphire substrates to the best of our knowledge. (C) 2007 American Vacuum Society.