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The breakdown voltages in unpassivated nonfield-plated AlGaN/GaN HFETs on sapphire substrates were studied. These studies reveal that the breakdown is limited by the surface flashover rather than by the AlGaN/GaN channel. After elimination of the surface flashover in air, the breakdown voltage scaled linearly with the gate-drain spacing reaching 1.6 kV at 20 mum. The corresponding static ON-resistance was as low as 3.4 mOmegamiddotcm 2 . This translates to a power device figure-of-merit V BR 2 /R ON =7.5times10 8 V 2 middotOmega -1 cm -2 , which, to date, is among the best reported values for an AlGaN/GaN HFET
Tipirneni et al. (Tue,) studied this question.