This paper describes the influence of surface roughness on the mobility of a - SiN : H and a - Si : H . For the a - SiN : H deposited by PECVD, the roughness was 0.832 nm. The a - SiN : H layer composed of (PECVD 150 nm + RACVD 100 nm) had better characteristic of roughness than the a - SiN : H layer (PECVD 250 nm) by 47%. The roughness of the a - Si : H (PECVD 200 nm) deposited on the a - SiN : H layer was 0.803 nm. And the roughness of a - Si : H (RACVD 100 nm + PECVD 100 nm) deposited the a - SiN : H layer is better than the a - Si : H (PECVD 200 nm) by 27%. After depositing the layer of a - SiN : H and a - Si : H at the best condition obtained by the experiments the mobility was measured. The a - Si : H is deposited by the PECVD, the obtained mobility was 0.218 cm 2/V ·sec. The a - Si : H was deposited by the PE/RACVD, the obtained mobility was 0.248 cm 2/V ·sec. The mobility is enhanced by 10% by depositing the a - Si : H layer using the PE/RACVD method.