Using ECR-CVD (electron cyclotron resonance-chemical vapor deposition), we can make amorphous-silicon (a-Si) and nanocrystalline (nc-Si) thin films. We are looking forward to improve the photo/dark conductivity ratio (σp/σd) by measuring the photo and dark current-voltage (I-V). In the ECR deposition, there are several factors which we can control and adjust for improved results, such as the amounts of silane and argon, the vacuum, and the temperature of the substrate. These become the critical factors for ECR deposition in order to make better films. Input gases consist of Ar, 2%SiH4 in He and H2. In the process, SiH4 is decomposed into SiHx. A residual gas analyzer (RGA) gives composition in the plasma. Because Ar possibly etches the substrate and Si is to be deposited, the best RGA signal is obtained with low Ar content. This work serves to correlate process conditions, RGA signals and electrical data. The best RGA signal occurs for 5 mTorr Ar, 60 mTorr SiH4:He, and power of 600 W. Best value of dark conductivity (σd) was 1.53 × 10-9 S/cm and1.58 × 10-5 S/cm for photo conductivity (σp and low value of σd indicate material with fewer defects. Adding extra H2 improves the photo-conductivity (σp). Applications of these films are heterojunction solar cells and thin film transistors. The heterojunction solar cell had a structure of metal grid/ 500°A of aSi:H/p-Si wafer/Ohmic contact. These cells gave an open circuit voltage (Voc) = 0.51 (V) and short circuit current density (Jsc) = 5.5 mA/cm2 under 50mW/cm2 tungsten halogen lamp. Thin film transistors using nc-Si, with gate length/width (L/W) =450/65 gave field effect mobility of 18 cm2/V-s, and Ion/Ioff of 1.25×105.