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

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High-quality C-doped GaN buffers with a very low doping concentration were grown on 2in. 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 (Lgd) of 16μm exhibited a high breakdown voltage (BV) over 1100V and low specific on resistance (ARDS(on)) of 4.2mΩcm2, 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 Lgd on device characteristics, it was shown that Lgd had a strong effect on ARDS(on) and BV while no noticeable change in maximum transconductance (gm,max) and maximum drain current (IDS,max) was observed when Lgd was varied. The ARDS(on) of a device [1.5μm gate length (Lg)] with Lgd>7μm was mainly determined by the gate-drain channel resistance. For a device (1.5μmLg)with Lgd<7μm, on the other hand, the ARDS(on) should be optimized by considering other important resistance components. The measured BVs increased with Lgd, suggesting that the actual device breakdown was determined by the gate-drain breakdown. The trend of the BV-ARDS(on) performance showed a clearly linear relation, suggesting that the device performance is very predictable with the variation of Lgd. 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.