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Wideband gap semiconductor GaN has received increasing attention for its potential for wide variety of high-power, high- performance switching and high-frequency devices application. In this paper the simultaneous annealing effect at a temperature of 400degC ~700degC in N2 ambient for 12 min on aluminum (1500 Adeg) as an ohmic contact while Pt/Ti (700 Adeg/700 Adeg) bilayer as a Schottky contact on the electrical characteristics of n- GaN Schottky diode was investigated. It was found that at a annealing temperature of 400degC produced the best (I-V) characteristics, barrier height (PhiB) 1.1 eV, ideality factor (eta) 1.1 as compared to the other annealing temperature. The (C-V) characteristics of n-GaN Schottky diode were measured at 100 kHz and 1 MHz frequency at different annealing temperature. It was found that at annealing temperature of 400degC, the depletion region is maximum with the capacitance value varied from 0.02 pF ~ 0.04 pF. At low frequency 100 kHz the capacitance increase with increasing forward voltage which is frequency independent, while at high frequency 1 MHz the capacitance- voltage curve is almost flat. The surface morphology of n-GaN Schottky diode before annealing and after annealing was observed by SEM, XRD. It was found that Al (1500 Adeg) didn't show any significant loss of dimensional stability at annealing temperature 400degC~700degC, while Pt/Ti (700 Adeg/700/Adeg) show balling effect, surface morphology degradation at above 400degC which was confirmed by XRD measurement. Hence we conclude that rectifying behavior of n-GaN Schottky diode was observed at annealing temperature 400deg C while annealing at above 400degC~700degC the rectifying characteristics of n-GaN Schottky diode changed to ohmic behavior due to Pt/Ti (700 Adeg/700 Adeg) island form of surface morphology occurred, while Al (1500 Adeg) as a ohmic contact show thermal stability at high temperature annealing above 400degC of n-GaN Schottky diode.