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To address the limitations of existing silicon solid-state photomultipliers (SSPMs), we are developing new photodetector elements in Al0.8Ga0.2As . With 80% aluminum concentration in the GaAs compound, the material effectively becomes a wide band-gap semiconductor with a band-gap energy of ~2.1 eV. The wide band-gap characteristic and the relative maturity of GaAs material processing makes Al0.8Ga0.2As an excellent material for developing improved SSPMs with lower dark current. Materials with larger band gaps have a lower limit in the associated thermally generated dark current. With a proper device structure, the AlGaAs SSPM is expected to provide a smaller dark current with high detection efficiency within the blue to UV spectral region, which is ideal for state-of-the-art scintillation materials, such as LaBr , CeBr , and CLYC that emit in the UV region. Utilizing commercially grown AlGaAs epitaxial wafers as the starting material, prototype photodiode elements have been designed and fabricated. The photodiodes, which have a mesa structure, exhibit a 20-30-V reverse bias breakdown. This work presents the fabrication and characterization of prototype avalanche photodiodes. Geiger pulses induced by both thermal electrons and photons are also presented, and the photon detection efficiency at low excess bias is estimated.