Large area p-i-n amorphous silicon-carbon heterojunctions with low conductivity and wide band gap doped layers were produced by plasma enhanced chemical vapor deposition at low temperature. Carrier transport and collection efficiency are investigated from dark and illuminated current-voltage dependence and spectral response measurements under different electrical and optical bias conditions. The results show that it is possible to control the absorption at a given wavelength and thus to tune the spectral sensitivity by changing the electrical bias around the open circuit voltage. A numerical simulation gives insight into the physical process and explains the bias controlled spectral sensitivity presented by the devices.