The development of high-speed dual-band photodetectors with high responsivity is important for several applications such as optical communication, biomedical imaging or spectroscopy. In this work, a phototransistor with ultra-high responsivity is demonstrated, which potentially also allows for very high bandwidths. The device is called graphene adjustable-barriers phototransistor and is potentially capable for dual-band detection in the visible-infrared (VIS-IR) range. A material combination of intrinsic hydrogenated amorphous silicon, graphene, and n-type germanium (n-Ge) is used for the demonstrator. The device operation is based on the light induced modulation of the graphene Fermi energy level and Schottky barrier heights. For the first time, the functional mechanism of the device is successfully demonstrated in the VIS range with responsivities exceeding 10⁷ A/W at a gate voltage of 20V. The bandwidth of the device is 1.2 kHz and is so far limited by the defective gate material hydrogenated amorphous silicon and relaxed feature sizes of the demonstrator. These results are an important step toward a new generation of high-responsivity high-speed photo detection devices.