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We have achieved a 9 ??m-thick AlGaN/GaN high-electron mobility transistor (HEMT) epilayer on silicon using thick buffer layers with reduced dislocation density (DD). The crack-free 9 ??m-thick epilayer included 2 ??m i-GaN and 7 ??m buffer. The HEMTs fabricated on these devices showed a maximum drain-current density of 625 mA/mm, transconductance of 190 mS/mm, and a high three-terminal OFF breakdown of 403 V for device dimensions of LgWgLgd=1.5/15/3 ??m . Without using a gate field plate, this is the highest BV reported on an AlGaN/GaN HEMT on silicon for a short Lgd of 3 ??m. A very high BV of 1813 V across 10 ??m ohmic gap was achieved for i-GaN grown on thick buffers. As the thickness of buffer layers increased, the decreased DD of GaN and increased resistance between surface electrode and substrate yielded a high breakdown.