Graded-gap Al x Ga 1 - x As/GaAs X-ray detectors with photovoltaic response have been designed and fabricated. A charge collection efficiency of 100% has been achieved in an Al x Ga 1 - x As layer with a thickness of 15μm without application of any bias voltage to the layer. Experimentally, the measured sensitivity achieves 0.9A/W. Amplification of the photocurrent takes place in the thin (15μm) Al x Ga 1 - x As layer, and an efficiency of 5x10 5 V/W is attained at an absorbed power of 10 - 7 W.The possibilities of using the new detectors for observation of X-ray images are considered.