The electrical and optical properties of doped hydrogenated amorphous silicon carbide alloys were studied. The activation energy for conduction, optical gap, and the optical absorption coefficients near band edge were investigated for the samples deposited by the glow discharge decomposition of silane-methan mixtures. The optical absorption coefficients near band edge for the undoped and phosphorous doped samples determined from photoconductivity data show the exponential form as α=α exp E/Eo with Eo = 0.07eV. However, by increasing doping concentration the absorption coefficients around 1.5eV show shoulder, which is caused by the increase of defect states by dopants. The optical gap for the a-SiC:H increase with the concentration of carbon in the material, and reach 2.0 eV for the boron doped a-SiC:H prepared by the decomposition of $SiH_4 + CH_4$. However, the photoconductivities for the a-SiC:H decrease with the carbon concentration since the incorperation of carbon in the material give rise to the defect states.
