Based on the temperature-dependent current-voltage (I-V-T) measurements and the temperature-dependent capacitance-voltage (C-V-T) measurements of Schottky diodes fabricated on n-type GaN，the mechanism of the electrical current transport was discussed using thin surface barrier (TSB) model. The experiment results indicated that there are different mechanisms at different temperatures and bias. Based on this assumption we give a modified I-V characteristic formula which gives excellent fit to the experiment data. The SBHs determined from high-temperature I-V curves，low-temperature C-V curves，and the metal work function agree well each other.