A process of forming a field effect transistor is disclosed. The process includes steps of depositing a first silicon nitride (SiN) film on a semiconductor layer by a low pressure chemical vapor deposition (LPCVD) technique; depositing a second SiN film on the first SiN film by plasma assisted chemical vapor deposition (p-CVD) technique; preparing a photoresist mask on the second SiN film, the photoresist mask having an opening in a position corresponding to the gate electrode; dry-etching the second SiN film and the first SiN film continuously in a portion of the opening in the photoresist mask to form an opening in the first SiN film and an opening in the second SiN film, the openings in the first and second SiN films exposing the semiconductor layer; and filling at least the opening in the first SiN film by the gate electrode. A feature of the process is that the opening in the first SiN film has an inclined side against the semiconductor layer and gradually widens from the semiconductor layer.