Fabrication and characterization of high breakdown voltage AlGaN∕GaN heterojunction field effect transistors on sapphire substrates

Abstract
High-quality C-doped GaN buffers with a very low doping concentration were grown on 2 in. c -plane sapphire substrates, and high-power Al Ga N ∕ Ga N heterojunctionfield 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 μ m exhibited a high breakdown voltage (BV) over 1100 V and low specific on resistance ( A R DS ( on ) ) of 4.2 m Ω 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 A R DS ( 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 A R DS ( on ) of a device [ 1.5 μ m gate length ( L g ) ] with L gd > 7 μ m was mainly determined by the gate-drain channel resistance. For a device ( 1.5 μ m L g ) with L gd < 7 μ m , on the other hand, the A R DS ( 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- A R 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 A R DS ( on ) were all enhanced to the point that high-power Al Ga N ∕ Ga N HFETs on sapphire substrates are now strong competitors for high-power switching applications.