Electron mobility of a hydrogenated amorphous silicon thin film transistor (a-Si:H TFT) a is important factor for a active matrix display device. The electron mobility of a-Si:H TFT is about 0.5 cm2/V•sec in current technology. If the electron mobility the of a-Si:H TFT increases to 1 ∼ 2 cm2/V•sec, TFT-LCD and OLED display can have drive IC within the panel. High-resolution display also can be made. Hydrogenated amorphous silicon nitride (a-SiN:H) thin films, as dielectric layer of hydrogenated amorphous silicon thin film transistor (a-Si:H TFT), were deposited by radical assisted chemical vapor deposition (RACVD) and plasma enhanced chemical vapor deposition (PECVD). Interface roughness between a-SiN:H and a-Si:H is important to improve electron mobility in a-Si:H TFT. We compared surface roughness of a-SiN:H thin films deposited by RACVD and PECVD and investigated field effect mobility of a-Si:H TFTs using a-SiN:H thin film deposited by RACVD and PECVD.