A hybrid-excitation technique was applied to nitrify amorphous silicon (a-Si:H) film surface using ammonia (NH3) gas and to deposit silicon nitride (SiNx) film using disilane (Si2H6) and NH3 gases for an a-Si:H thin-film transistor (TFT) at 280°C. With the use of this SiNx film as a gate insulator, a coplanar a-Si:H TFT with high field-effect electron mobility (5.3 cm2/(V·s)) was fabricated. This high electron mobility is considered to be due to three features as follows: (1) a good SiNx/a-Si:H interface created by the hybrid-excited nitrification of the a-Si:H film surface, (2) an ion-bombardment-free nitrogen-rich SiNx film fabricated by hybrid-excitation chemical vapor deposition (CVD) and (3) an a-Si:H film with less ion-bombardment damage. This hybrid-excited nitrification and the CVD effects on electrical properties of SiNx/(100)Si interface are also discussed.