A new npn Al-doped zinc oxide (AZO)/GaN heterojunction bipolar transistor (HBT) is presented. The fabricated HBT with an emitter size of 120×120m2 exhibits a current gain of 5.8 at V BE=3.0V. The anomalous high current gain is observed at low current level due to the leakage current. The common-emitter output characteristics demonstrate DC current gain of 1.2. In addition, improved device characteristics are observed after the P2S5/(NH4)2S treatment.