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[Display omitted] • High-quality Ga 2 O 3 heterostructures grown via LMBE with lattice/energy band matching. • Modulating APDs' voltage and dark current by adjusting MgO layer thickness. • The impact of the barrier layer on the nBn structure APDs performance was systematically studied. • The fabrication of Ga 2 O 3 -based solar-blind APDs with the current optimal performance has been achieved. Ga 2 O 3 -based solar-blind avalanche photodetectors (APDs) offer advantages such as compactness, low power consumption, and high stability. They could improve the integration and reliability of solar-blind photodetectors, and serve as a viable substitute for photomultiplier tubes (PMTs) in deep ultraviolet signal detection. We have previously demonstrated that the n-Barrier-n (nBn) structure, composed of Ga 2 O 3 /MgO/Nb:SrTiO 3 heterostructure, can efficiently reduce the dark current and enhance the reverse breakdown voltage due to the increased conduction band offsets. The barrier layer is a key factor in determining the device performance in the nBn heterostructure. Hence, we further optimize the study by varying the thickness of the barrier layer MgO to examine its effect on the devices. This modulation can affect the avalanche multiplication process, and thus enable the tuning of APDs' performance. In this paper, we present the optimized thin film growth process, and the systematic investigation of the responsivity and gain of the device with different barrier layer thicknesses. The optimal performance was achieved with 25 nm MgO thickness. We also explore the underlying mechanism to elucidate the role of the barrier layer. Our results provide insights into the influence of barrier layer thickness in the nBn structure. [ABSTRACT FROM AUTHOR]