A process of forming a silicon nitride film on a nitride semiconductor layer as a passivation film is disclosed. The process first sets a temperature lower than 500° C. to load into a growth reactor, a wafer that provides the nitride semiconductor layer thereon. Then, the process raises the temperature to a deposition temperature higher than 750° C. while replacing the atmosphere in the reactor with pure ammonia (NH3), or a mixed gas of NH3 and N2 with a NH3 partial pressure greater than 0.2, and sets the pressure higher than 3 kPa. Finally, with the pressure lower than 100 Pa and di-chloro-silane (SiH2Cl2) supplied, the SiN is deposited on the nitride semiconductor layer.