Forward-current-density J F /forward-voltage V F 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 μ n was used as a parameter to fit the J F - V F characteristics of both reported and fabricated GaN SBDs. At 300 K, μ n was fitted to be 600 cm 2 /V ·s when electron concentration n was 1 × 10 16 cm -3 and 750 cm 2 /V ·s when n was 5 ×10 15 cm -3 . Accordingly, the theoretical μ n - 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 J F - V F 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.