In this article, we investigate a large area heterostructure obtained by growing a p-i-n amorphous silicon junction on a p-type crystalline silicon wafer. This stacked structure behaves as a back-to-back diode if the thickness of the n amorphous layer is thick enough to avoid a complete depletion of the layer. The p amorphous layer is the window layer for incident light. This device is able to discriminate visible and near infrared radiations simply by varying the bias conditions. Due to the difference in the absorption coefficient of amorphous and crystalline materials, when the applied voltage reverse biases the amorphous diode and forward biases the heterostructure diode, the visible spectrum is detected, whereas in the opposite bias condition, the near infrared spectrum is detected, as only a higher wavelength can reach the rear diode. We characterised this device performing steady-state and transient measurements of photocurrent in different conditions of incident light wavelength and bias voltage. An analytical model for the photocurrent is used to optimise the thicknesses of the amorphous layers and numerical simulation with SPICE is performed for steady-state and transient measurements in order to evaluate the possibility of integration in a matrix.