Co/n-GaN SDs has been prepared by magnetron DC sputtering technique. The Co/n-GaN SDs have annealed at 600 °C after a post-deposition. The diode parameters such as the ideality factor, barrier height and Richardson constant have been determined by thermionic emission (TE) equation within the measurement temperature range 60–320 K by the steps of 20 K in the dark. It has been seen that the parameters depend on the measurement temperature indicating the presence of a lateral inhomogeneity in the Schottky barrier. Therefore, it has been modified the experimental data by the thermionic emission (TE) mechanism with Gaussian distribution of the barrier heights by using Tung’s theoretical approach that the Schottky barrier consists of laterally inhomogeneous patches of different barrier heights. Thus, the modified Richardson plot according to Tung’s barrier inhomogeneity model  has given a Richardson constant of 27.66 A/(cm2 K2).