Requirements for continuous operation of photodiodes in the presence of nuclear radiation has led to the development of a novel AlGaAs/GaAs photodiode. Device structure and semiconductor properties of this design were optimized to balance the need for photodetection at &lgr;=820 nm against unwanted excess photocurrents induced by ionizing radiation. To achieve radiation hardness the photodiode employs a double‐heterojunction device structure. The spectral response of this detector, 1 keV–10 MeV, may offer unique opportunities for use in high‐temperature plasma diagnostics compared to typical bare Si:p‐i‐nx‐ray photodiode characteristics. Application of this AlGaAs/GaAs detector in a fiber‐optic link will be reviewed. Also, use of this detector in simple atomic‐absorption‐edge filtered x‐ray detector channels is presented.