High-quality C-doped GaN buffers with a very low doping concentration were grown on 2 in. c-plane sapphire substrates, and high-power AlGaN/GaN heterojunction field effect transistors (HFETs) on sapphire substrates for high-power switching applications were fabricated using a self-align process. The fabricated devices with gate-drain spacing (L-gd) of 16 mu m exhibited a high breakdown voltage (BV) over 1100 V and low specific on resistance (ARDS((on))) of 4.2 m Omega cm(2), with no additional photolithography process for a field plate design. This result approaches the SiC theoretical limit and is a record achievement for GaN-based HFETs on sapphire substrates, to the best of our knowledge. Based on the investigation of the influence of L-gd on device characteristics, it was shown that L-gd had a strong effect on ARDS((on)) and BV while no noticeable change in maximum transconductance (g(m),(max)) and maximum drain current (I-DS,(max)) was observed when L-gd was varied. The AR(DS(on)) of a device [1.5 mu m gate length (L-g)] with L-gd > 7 mu m was mainly determined by the gate-drain channel resistance. For a device (1.5 mu m L-g) with L-gd < 7 mu m, on the other hand, the ARDS((on)) should be optimized by considering other important resistance components. The measured BVs increased with L-gd, suggesting that the actual device breakdown was determined by the gate-drain breakdown. The trend of the BV-AR(DS(on)) performance showed a clearly linear relation, suggesting that the device performance is very predictable with the variation of L-gd. As a result, with improvements in the material quality of a GaN buffer on sapphire substrate, the off-state BV and ARDS((on)) were all enhanced to the point that high-power AlGaN/GaN HFETs on sapphire substrates are now strong competitors for high-power switching applications. (c) 2006 American Vacuum Society.