Thin avalanche layers have been adopted to achieve low excess noise and high-gain bandwidth products in InP and InAlAs avalanche photodiodes. In this paper, we report the excess noise characterization in a series of Al_1-xGa_xAs_0.56 Sb_0.44 (x = 0, 0.05, 0.1, 0.15) diodes with avalanche layer thickness of 110-116 nm. These alloys, lattice matched to InP, showed lower excess noise than InP and InAlAs. Dark current, most probably originating from surface leakage, was observed to be lower in composition with higher Ga concentration. Avalanche gain and excess noise measurements using lasers of 543 and 633 nm wavelengths indicated that at a given electric field, the electron ionization coefficient is larger than the hole ionization coefficient. Using the 543 nm laser, low excess noise data corresponding to an effective ionization coefficient ratio of k = 0.1 in the conventional excess noise theory was measured in Al_1-xGa_xAs_0.56Sb_0.44 (x = 0.05, 0.1, 0.15), although pure electron injection was not achieved. Our results demonstrated the potential of using Al_1-xGa_xAs_0.56 Sb_0.44 (x = 0.05, 0.1, 0.15) as replacement for InP and InAlAs for high speed and low excess noise avalanche photodiodes. The data reported in this paper is available from the ORDA digital repository (https://doi.org/10.15131/shef.data.5155822).

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