p-i-n AlxGa1-xAs/GaAs detectors with graded compositions and graded doping were grown and prepared. From the current–voltage and capacitance–voltage measurement results, the devices had good p–n junction diode characteristics, and the electric field strength under an unbiased voltage was 1.7 9 105 Vcm-1. The full width at half maximum and charge collection efficiency of the detectors obtained from energy spectrum measurements of 5.48-MeV alpha particles were 3.04 and approximately 93%, respectively. In this study, we created the most advanced and promising state-of-the-art unbiased detector reported to date.