The experimental results for different structures of contact image sensor are reported in this paper, which includes Schottky barrier, p-i junction, p-l-n junction, and MIS structures. The J-V characteristics of p-i-n a-Si: H contact image sensor under dark and illuminated conditions have been simulated by solving the Poisson's equation and the continuity equations, and the results are correlated with the experiments. The dependence of the dark and photo currents on the parameters such as the density of states in the gap, intrinsic layer width, dopant concentrations of p
* layer and n
+ layer are discussed.