The invention relates to a photodetector with at least one first and at least one second semiconductor layer. Means for applying a voltage to the semiconductor layers are provided. The voltage can be set within a first voltage range so as to generate an electrical field confined essentially to the first semiconductor layer for the transportation of photogenerated charge carriers. The upper limit of the first voltage range depends on the layer thickness and on the material used. When the voltage is set within a second range, for example a range of higher voltage than the first range, the electrical field also extends into the second semiconductor layer or other semiconductor layers for transporting photogenerated charge carriers. This means that by varying the applied voltage and maintaining the intensity of the irradiated light the number of photogenerated charge carriers is increased and consequently the photocurrent rises discontinuously. The sensitivity of the photodetector can thus be controlled relatively easily and effectively. In another advantageous embodiment of the invention the first and second semiconductor layers have different spectral sensitivities. This means that the spectral sensitivities can be modified and adjusted by varying the applied voltage. Depending on the semiconductor layers present, different semiconductor layers contribute to the photocurrent and the sensitivities are added together. Thus, depending on the materials chosen, the sensitivity can have a marked spectral dependency, in particular exhibiting different patterns according to the voltage.