In this work, the optimization of reverse leakage current (I_R) and turn-on voltage (V_T) in recess-free AlGaN/GaN Schottky barrier diodes (SBDs) was achieved by substituting the Ni/Au anode with TiN anode. To explain this phenomenon, the current transport mechanism was investigated by temperature-dependent current–voltage (I–V) characteristics. For forward bias, the current is dominated by the thermionic emission (TE) mechanisms for both devices. Besides, the presence of inhomogeneity of the Schottky barrier height (qφ_b) is proved by the linear relationship between qφ_b and ideality factor. For reverse bias, the current is dominated by two different mechanisms at high temperature and low temperature, respectively. At high temperatures, the Poole–Frenkel emission (PFE) induced by nitrogen-vacancy (V_N) is responsible for the high I_R in Ni/Au anode. For TiN anode, the I_R is dominated by the PFE from threading dislocation (TD), which can be attributed to the decrease of V_N due to the suppression of N diffusion at the interface of Schottky contact. At low temperatures, the I_R of both diodes is dominated by Fowler–Nordheim (FN) tunneling. However, the V_N donor enhances the electric field in the barrier layer, thus causing a higher I_R in Ni/Au anode than TiN anode, as confirmed by the modified FN model.