In this work, we designed and optimized a gallium nitride (GaN)-based betavoltaic (BV) cell using an AlGaN back-barrier layer and finger structure for improving the output power density. A short-circuit current density (J(SC)) and an open-circuit voltage (V-OC) of the BV cells associated with an output power density were investigated by using electron-beam (e-beam) irradiation. The device with the Al0.25Ga0.75N back-barrier layer exhibited an enhancedJ(SC)because the potential barrier with a high height reduced excess carriers moving to the substrate region. The finger structure of the proposed BV cells was optimized by changing parameters such as the width of the intrinsic GaN region (Wi-GaN) and heights of the p-GaN and n-GaN regions (H(p-GaN)andH(n-GaN)). The optimized BV cell with aW(i-GaN)of 100 nm, aH(n-GaN)of 100 nm, and aH(p-GaN)of 200 nm obtained a higherJ(SC)compared to that of the conventional p-i-n BV cell because an optimum structure resulted in a wide depletion area, which was involved in the improved charge collection. As a result, the output power density of the proposed BV cell was enhanced by 14.8% than that of the conventional BV because of the improvedJ(SC). The proposed structure shows a high potential for BV cells with a high-power conversion efficiency.