GaN PiN diodes with a 4 μm Si-doped n--GaN drift layer (n~7×1016 cm-3) were grown on free-standing GaN using metalorganic chemical vapor deposition. Atomic force microscopy showed smooth surfaces with a step structure indicating good 2D growth. The dislocation density and impurity incorporation in the drift layer were remarkably reduced compared to a similar diode structure grown on sapphire. The full width at half maximum of the (0002) rocking curve was 79 arcsec, much smaller than 230 arcsec for the heteroepitaxial structure. The diodes on GaN demonstrated rectification up to –265 V, corresponding to a critical electric field ~2.7×106 V/cm. The maximum value of the figure of merit is ~2.4 MW cm-2, which represents a 2.2× improvement over the diodes on sapphire.