Photodiodes are devices used as image sensors, reactive to polychromatic light and subsequently color detecting, and they are also used in optical communication applications. To improve these devices performance it is essential to study and control their characteristics, in fact their capacitance and spectral and transient responses. This study considers two types of diodes, an amorphous silicon pin and an amorphous silicon carbide pin, whose major characteristics are simulated, using the AFORS-HET program .The pin diode structure can be defined using contacts, interfaces and optical layers and then common measurements can be simulated by a numerical model, the AFORS-HET program. I-V, C-V characteristics, spectral response are simulated for both devices, without and under different illumination wavelengths. The results will allow a comparison between the main properties of amorphous silicon and amorphous silicon carbide diodes. We can conclude that sinusoidal frequency varies capacitance values as well as incident light wavelength. And when carbon is included in an amorphous silicon diode structure, its electrical and optical properties change.