Using　GaN-on-Si　epilayers,　for　the　first　time,　fully　vertical　p-i-n　diodes　are　demonstrated　after　Si　substrate　removal,　transfer,　and　n-electrode　formation　at　the　top　of　the　device.　After　SiO2　sidewall　passivation,　the　vertical　p-i-n　diodes,　with　n-GaN　facing　up,　exhibit　V-ON　of　3.35　V　at　1　A/cm(2),　a　low　differential　ON-resistance　of　3.3　m　Omega　cm(2)　at　300　A/cm(2),　and　a　breakdown　voltage　of　350　V.　The　corresponding　Baliga＇s　figure　of　merit　is　37.0　MW/cm(2),　a　very　good　value　for　GaN-based　p-i-n　rectifiers　grown　on　Si　substrates.　The　results　indicate　that　fully　vertical　rectifiers　using　GaN-on-Si　epilayers　have　great　potential　in　achieving　cost-effective　GaN　devices　for　high-power　and　high-voltage　applications.