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Forward-current-density JF/forward-voltage VF characteristics of vertical gallium-nitride (GaN) Schottky-barrier diodes (SBDs) and p-n diodes on free-standing GaN substrates were computationally, as well as experimentally, investigated. Based on the thermionic emission model, electron-drift mobility μn was used as a parameter to fit the JF-VF characteristics of both reported and fabricated GaN SBDs. At 300 K, μn was fitted to be 600 cm2/V ·s when electron concentration n was 1 × 1016 cm-3 and 750 cm2/V ·s when n was 5 ×1015 cm-3. Accordingly, the theoretical μn-n curve for a carrier compensation ratio of 0.90 was applied in the case of n-GaN layers on GaN substrates. With respect to GaN p-n diodes, the reported JF -VF characteristics were successfully fitted with dislocation-mediated carrier lifetimes in the high-injection region and with Shockley-Read-Hall lifetimes in the generation-recombination current region.