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Although alpha-voltaic cells have shown great potential in unattended miniaturized systems for compact, long-lifetime and independence of external energy input, the power conversion efficiency of state-of the-art alpha-voltaic cells is still much lower than the theoretical limit. Here, an alpha-voltaic cell based on a gallium nitride transducer with PIN structure is designed and investigated. We find that isoelectronic aluminum-doping is an effective way for boosting the performance of the gallium nitride transducer by decreasing the unintentional doping concentration, deep trap concentration, and dislocation density in the gallium nitride epilayer. The isoelectronic aluminum-doped cell demonstrates a large depletion region of 1.89 μm and a charge collection efficiency of 61.6% at 0 V bias, resulting in a high power conversion efficiency of 4.51%, comparable to the best gallium nitride beta-voltaic cells. This work increases alpha-voltaic cell efficiency, bridging the gap between nuclear micro-batteries and real applications in extreme environments.